I made two animated GIFS to show what's going on in two separate places:
(the flash "stage" is set to 400% in order to show them better; in reality they are 4x smaller, but are very noticeable)


As you can see, some weird, white geometric artefacts have popped up for no reason that I can discern. I don't even have any vector shapes in my file; it's all made up of GIFs, JPGs and PNGs.
These shapes appeared after I added a layer that I guess takes up a lot of system resources.
Anyway, I'm just wondering if anyone else has seen something similar.
I'm thinking, would it work if I just drew some black shapes on top, to cover them up...?
EDIT: Ok, I tried that... it seems to cover them up in the Flash preview, but some are still left when I actually make a swf file.