First off: specs. Standard edition, Windows 64bit, on a Macbook Air (yes, Windows on an Air). Video card is a Nvidia GeForce 320M.
I've been working on a platform game where you have a jetpack. Since platform and physics don't play nice together, I couldn't use forces for the jetpack, so I just decided to adjust gravity instead.
What I noticed, though, is that if you set gravity to, say, -300, when SPACE is pressed, and then have another event to set gravity to 300 when SPACE is not pressed.
What I find is that the not pressed event never picks up, as shown in the .capx below.
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This bug only occurs, though, when the values are exact opposites. If you set gravity to -299 on SPACE pressed, and 300 when not pressed, there is no problem, and we get the expected behavior. It's almost like there is a check for identical value before the system tries to swap gravity, but the check is only comparing absolute values.
Hope this helps!