Whats are the most common problem you all encountered?

For the past 7 week of my game development on flash, I have encountered several tough problems. Most of them are animations and collision detections.

For example, my animations crash at certain point of the game… Just because i forget to set a boolean to false in my game. That cause my animations to enter an endless loop. But after i found out that many other people encounted the same problem as me.

So I was wondering what are the most common problems you guys out there encountered? Maybe we could share and discuss if there are any possible solutions?