As3 flash project targetting flash player 10 error 1172

Question

This is my first post so please excuse me if the answer is trivial.
I am creating a simple project where a movie clip contains a motion tween lasting 180 frames.
So I create a tween layer and create the animation.
I now create another layer and perform a stop(); on frame 30.
When I compile or test,  it produces loads of 1172 errors where common classes (like vector) cannot be found. 100 errors!
This is targeting flash player 10. when I target player 9 all works.
have things changed that dramatically with flash player 10?
what are the consequences of continuing with flash player 9? I prefer to fix the problem and enhance my knowledge.
Thanks guys.

Answer 1

If it's playing in FP8, it can't be AS3.  Generally, AS2 compilation doesn't throw a fit like AS3 compilation does, so seeing errors in AS3 is more likely.  It sounds like you my be using AS2 stuff with an AS3 setting until you set it for FP8 where the publish settings will automatically adjust to AS2.  Beyond that, FP10 has had some issues that FP9 hasn't.