Flash has so many weird odd bugs it’s sickening.
I was working with a simple timeline animation in Flash that looked like this:

On the timeline was a shape tween that was acting as a mask for two MC’s. I had some initialization code that I only wanted to fire once contained in one of the MC’s, but for some reason it was called twice because the MC was being instantiated twice. I couldn’t figure out why the MC was being instantiated twice, so eventually I took a shot in the dark and converted the shape in the shape tween to a symbol. It worked and the code only fired once. So in the end my timeline looked like this: