Switching from AS2 to AS3, i had this weird problem, that was really a pain in the ass. really easy the solution, but you need to know 
the Problem is easy. i have two movieClip on the Stage. mc1 and mc2. mc2 is partly on mc1. in AS2 if you put a RollOver ( for example ) on mc1, you get the RollOver event on ALL the shape of mc1, in both case : there is some MC on mc1 or not.
in AS3 the mc2 cover the firing of the RollOver Event. it means that you get the RollOver only on the Real RollOver of the mc1. easy solution ( and obviously much better as you can detect the shapes of other MCs )
you need just to use
mc2.mouseEnabled = falseif there are other MCs in mc2 you need to write
mc2.mouseChildren = falsethis happens as Event Bubbling was finally introduced with AS3