Missing/empty symbols, "Could not load scene into memory. Your document may be damaged." CS6

Question

Hey folks,
we've been getting this problem a lot. We had it in Flash CS5.5 and still have it CS6.
We're animating a series in Flash and this problem keeps cropping up way to often, but seems totally random.
Basically I'll be working on a .fla saving regular versions through out the day, no problems. I'll close the file down. Then I (or someone else on a different computer) will open it up and I'll get the error msg "Could not load scene into memory. Your document may be damaged." Everything will be fine except for a few missing graphic symbols. Sometimes just the one, sometimes a whole bunch. So where the symbol should be on the stage, instead you get a small white square, but it still contains the animation information. So it will still move about the stage. If I bring in the same symbol from an old scene I can swap them out and the problem is fixed, (until it does it again on a totally different graphic symbol).
If i select the symbol in the library the preview is white/blank and i can't go into it to edit. When I select the symbol on the stage, the "instance of:" gives me a blank "_ _ _" in the properties tab. How ever if i right click show in library it does show me the correct (although broken as in blank) symbol in the library.
I havent been able to reproduce this problem on purpose but I'd say it happens in roughly one scene out of 10. Its seems to happen at random, I know there must be something in common but i havent figured out what. Its going on in heavy scenes (large library multipule characters), lite scenes (just the one character). Its not like it happens to the same symbol, different ones each time. Everything is local to the scene, all elements are created in flash. We are working across a network but as everything is local to the scene i don't see how that would be a problem. Also we don't have duplicate named symbols. It also won't necessarily affect something that has been changed since the last version. could be a background element thats been there, untouched since the first version of the scene.
One point on how we set up our scenes which may be relevant. Start with empty scene, copy and paste symbols from other scenes to populate. Save.
The problem could appear on say version 4 or version 36. Older versions are usually fine.
So to sum up. Flash file is fine when we save it, open it up again later and a symbol will have disappeared. It's happening far too often.
I know how to fix the problem, thats not what I'm asking. I need to know WHY its doing it so we can stop this from happening in the first place.
Phew! Thanks for reading this far. Any help would be highly appreciated.
Sander/

Answer 1

Can you elaborate on how you use the network? Are you working from FLA/XFLs stored on a machine across a network (and saving them across the network)? I had plenty of those issues and Adobe has always warned not to work across a network. I just fell into the habit of copying over what I need to work on locally, then updating the file servers at the end of the day. Nothing was corrupt after that.
Also lately in CS5.5 (not CS6 yet) I had noticed that I could change some graphics assets, close the document in OS X and open in Windows only to find freshly updated graphics reverted back. What's even more odd is if the OS X machine that made the changes opens it the changes are still made. This happens vice versa as well. My only solution on that was not to work cross-platform with other machines on a network unless absolutely necessary.