Common mistakes (discussion)

Let’s talk about our common mistakes that we all make.

I’m not disciplined enough with my variable names. I like to use camelCase but I’m inconsistent and I don’t always capitalize the first letter. This can cause issues.

For colors like “255;0;0” I’ll use commas instead of semicolons.

I frequently add a group but forget to add any objects to it.

I still have trouble with object picking especially when it’s an opposite or inverted conditions. It can be difficult to know when to invert or use NOT.

Sometimes, the order of objects in a collision condition matters. It’s not often but there are times like when inverting it.

I still have problem knowing when a for each object is needed. GD is surprisingly intuitive at times. Other times, certain expressions will only reference 1 object instead of the expected object. It’s usually because multiple objects are picked and some actions can’t handle multiple objects and GD can’t figure out which object that you want.

Let’s hear yours so we can all learn.


I don’t know what you think…
but to my eyes 80% of the issues newcomers posts here about performance,crash,undesired physics/forces,unworking vars, ecc… are caused by an improper use of trigger once.

Speaking for myself most of the times i go to fast on what i have in mind then i have to go back again to turn everything in a FSM…losing days to fix everything…and isolate events


This is a great idea for a collective discussion. It’s too bad it can’t be pinned. I’m sure I’ll be adding a lot of common (and uncommon) mistakes I catch myself doing.

One thing I tend to do, less so now that I watch myself: When I am creating objects that will all have the same variables, I make the first object and add all the variables needed to the objects variable panel. Then I duplicate it enough times to make the rest of my objects. But I am always so focused on variables that I often forget the objects will also need a certain behavior or effect, and I forgot to add it to the initial object that I used for duplicating, and have to go back and add it to all the duplicates too. So much for saving time!

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Yes, trigger once is a common mistake. Sometimes, it’s used too often and other times not enough.

Another one is states. A lot of people create a domino effect by using an event that changes a value which makes the next condition true and undos the first action.

Variable = false set Variable to true
Variable = true set Variable to false

How to make toggleable states

Another one is using AND where it’s not needed.
Events all need to be true by default. The only place you need an AND is inside an OR

One of these is true (OR)
----Mouse is down
----All of these are true (AND)
-------Mouse is Released
-------Mode equals true

Either the mouse is down or the mouse is released and mode is true.


Another common mistake is putting conditions inside a loop event like a for each object This doesn’t always cause an unexpected behavior but does affect the efficiency. It’s extra important to be efficient when every millisecond counts.

Examples <click to open>

For each object
----mouse is down

While this will work, it’s checking the mouse condition each time. The mouse state won’t change during the loop or even the frame.

A more efficient way is

Mouse is down
----for each object

Now, the loop won’t happen if the mouse is released and when it is down, it will be only be checked once not one time for each objects.

Another involves object filtering.

For each object
—object is visible

This will go through the loop and check each object separately.

Object is visible
----for each object

This will pick the visible objects using 1 condition and then use that list to loop through only the visible objects.


That For Each Object usage example is spot on. Reduce the working set of objects, then iterate.

The same can be applied to multiple conditions in one event. Order them by the conditions that either reduce the list or working set of objects the most, or that are least likely to occur.


Most cardinal mistakes i see other users make
At beginning of the scene
Which ppl believe mean
Start at beginning and keep going
Where it means start ONLY at beginning

Other is not understanding that event is meant for condition and action to run
And not be bunch of both and engine will magically understand what you want to achieve by doing this

Where my own mistakes are
I often copy paste Object.CenterX() to not type it again for Y
So i copy paste it to Y and change X to Y
Yet i do not sometimes rename X to Y
And i end up with Object.CenterX() Object.CenterX()
I was victim of this mistake way too many times
And it happens to me even now and not just when i was starting


A critical but deceptive mistake that I used to do a lot before and I see many people fall for it, is using “trigger once” inside a For Each Object event.

It may seem like it makes the event triggers once for each object. But what actually happens is that if the conditions are met for any of the objects, the event triggers only once and stops after that. It’s confusing even when trying to explain it!


“At the beginning” is a little vague. I think I might have originally thought that. It also helps to put all of the “At the beginning” events at the beginning under a single event. There’s no need to mix it in with other events since it only executes once. It makes the rest of the events leaner and easier to read.

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Another commonly misunderstood action is wait . I originally thought it caused the entire project to pause instead of just the child events and sub-events.

Honestly, I personally try to avoid wait. It has it’s uses but often times I prefer to use a timer.
A lot of events also have their own completed conditions like tween, animation and sound. It’s more efficient to use them than it is to guess how long it might take. wait does make for a cleaner and more streamlined event sheet but it can cause unexpected actions and once set you can’t delete waits.

Also, you need to make sure the waits don’t build-up. It’s important to use some form of trigger once. Otherwise, you can have 60 waits added every second and x seconds later they all start to expire. Sometimes, wait is more like a delay if you don’t use some form of trigger once. You have the initial wait and then a constant release of additional waits like a red light turning green and all of the built-up cars being released.

Examples of tween, animation and sound

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Here’s a link to an explanation about how the trigger once works (and excuse the poor repeat for each object events - it was written before I knew any better)


Also sometimes is better to use the repeat 1 time event instead to trigger once something that needs to not be triggered just once…to me it happened when using the homing bullet extension

I’m honest i’ve this in my project.
I used it to check a fast collision eg when a whip it the enemy to create a fast combo (bcs the whip was ofc still on the enemy the next step)

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I’m not sure how Repat 1 event would be any different than just using a normal event, if the Repeat 1 event is triggered on every frame or repeated for each object.

i’m gonna post a screenshots here , maybe you gonna notice something i have not…
but in short words i have a repaet for each object event…
now … if you use trigger once in a subevent that events will works for one instance of that object…will it works instead with the repeat one event cos it will pick all objects cos there is no trigger once…
i’ve a conditions that works only like this…(now that i remember better i think it was cos a timer, i binded a timer for each enemy than i noticed that with trigger once only 1 enemy was shooting an homing bullet to player at chosen time, than i tried with repeat 1…issue gone)

I’m a bit confused myself. How is a repeat any different than just not having any conditions? Or a parent event to the for each object that has some form of trigger once. That way it prevents the for each object from being triggered multiple times.

Unless, I’m missing something.

I don’t know…maybe it was cos i had an extension action in the event…gemme time . when i’m back home i’m gonna post a screen. it may help to improve my game.


Ok so keep in mind that you may notice bad things since it’s a test version…
anyway i’ have this external events called Ai which is the base to trigger different enemies state.

Now this is where i noticed that enemies were not attacking the player in a randomized way. I tried conditions with trigger once a lot without success. (only one enemies and 1 timer were taken into consideration)
With repeat 1 everything works fine i guess because timers aren’t influenced or probably bcs something written in the homing extension that i don’t want to dig now… or probably cos the trigger once is already called by the previous event
I tricked it cheking the animation. so if the animation not end it will not consider the timer and trigger the bullet while still in the same animation. I don’ know, maybe it’s hard to explain…
I had so many problems about this that i decided to add a text above enemies to check timers and ai value.

HAHAHAHAHAHAHA, I’m sorry, Keith, but this is so funny! :joy:

Same here. I even created a topic with this question asking for some help.

I always forget to mark the scene whenever I open Gdevelop and every first preview is in the wrong scene.

I keep finding old things in my code which leads me to believe that when I make changes to the code I always forget something…

Sometimes I click wrong and close the scene’s events tab and have to open again and drag the events tab next to the scene tab.

I don’t know how to write exactly expressions involving arrays and variablechildcount. I always need to copy from somewhere else in the events or I’ll end up doing it wrong.

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Ok here’s one. If you are using a Text Input object, do not set up a Key Released event such as “Return key released” to trigger a set of actions supposed to occur after the player is done inputting text.

If you do not use Text Input objects much, you will probably spend 10 frustrating minutes trying different methods and changes to figure out why your variables and save events are not working. You will just be on the verge of concluding that GDevelop is broken, when you will realize that you are the one that is broken, because with the Text Input still in focus, of course the key release is not even registering because it’s counting as text input. You will then click the mouse somewhere else in the preview to take the focus off the Text Input, hit the return key, see everything working and feel that this was not one of your finest moments.

I also struggle with variable case too.

That’s why I love development interfaces in which variables are not case sensitive, but adaptable.

As an example, Visual Basic in Visual Studio (I don’t remember now if C# works the same in Visual Studio, or which ones does it in Visual Studio Code). Not the best or most versatile language, but I love how you can write (name) the variable with any case, and the engine will automatically understand it’s the same variable.

It even automatically updates the case for all other instances of the same variable based on how you write it the last time you name it (oganizationalIy wise, I cannot think of a good reason to keep different variables with the same spelling, but different case, in the same project).

That makes the variable system so precise and fast to work with it; but flexible, functional, and accessible to all kinds of users.

Also, there you can choose if variable declaration is strict or not.

In a beginner-friendly but powerful system, that also should apply to the use of commas and semicolons in situations like RGB color codes.

I have struggled with this too. Sometimes I’ve discovered a ‘For each object…’ loop is not needed since the actions seems to be applied to all instances of an object, even if I have just make a reference to a specific instance before in a condition or an action of the same block.

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