Why does layer effect have a glitch?

The pixelate effect shows a visible glitch when it starts as per the screencast.

It’s programmed to start as soon as lena and francis go outside the screen. But when it starts, there’s a visible glitch in the screen.


UPDATE: The screen cast doesn’t seem to catch the glitch

UPDATE: the cause was the pixel size—it starts at 0. When I changed it to TimerElapsedTime("goodbye")*5+1 (add 1 to ensure it starts at 1), the glitch went away.

You can see the original bad behavior in this preview—just keep touching the screen until you get to the end of the second scene (page 1).