I've been pulling my hair out for about an hour with this beauty, and after finding the solution I thought I should share here, if for no other reason that I know I'll get it again and lose another hour ( I'm 36, my memory is shot ).
When trying to make a new bitmap instance of a bitmap in the library, along the lines of:
private var bouncerBitmap:Class;
var bm:Bitmap=new bouncerBitmap();
I was getting invalid data when running the swf, basically it was failing big time.
Had a bit of a google and found this. For some beautiful unknown reason, sometimes when you import a png to the library and set the compression to jpg ( Although in this case I actually hadn't, Flash just took it upon itself to make it a jpg for me ) it screws up.
Anyway all fixed now, and I've even got this as a reminder for next time.