Personally I think this is a bug, but maybe there is some logic to why it happens?
Make a new FLA file,
Draw something simple on stage and make it a movieclip.
Give it the instance name bug.
Make your stage timeline 10 or so frames long
In the first keyframe just stick in this code :-
bug.x=Math.random()*550
bug.y=Math.random()*400
addChild(bug)
If you don't include the last line 'addChild(bug)', everything is fine.
If you do include it, each time the timeline loops, a new instance of 'bug' is created and positioned, meaning ultmately you get loads of bugs.
This just seems very norty to me.
Admittedly having a need where you need to 'addChild' something that is manually placed on stage could also be considered very norty.
David