Okay, I'm sorry to have to make a thread about this, usually I'm quite fine with finding the answers to issues with google. But with this I'm totally stumped.
Sometimes when I undo things when working with edit poly, something strange happens. A bunch of verts delete themselves, normals get broken, verts moved...
There is never a way to undo this. I have to reload my last save. Today I loaded the object I was working on to find it like this:

which was certainly not how I left it. If I delete the most recently edit poly, the state is fine again.
I have absolutely no idea what is causing this. I've been using 3ds max for maybe 9 months now, and this is only recently started happening.
I'm using 64-bit 3ds max 2012.
I would be really appreciative of help, it's getting really annoying having my work destroyed

Replies
A) If you change anything in the lower stacks of modifiers, it will break the rest. So for example, changing your polygons in Edit Poly 2, and then changing some stuff in Edit Poly 4 will lead to such issues.
I suggest getting all the Service Packs and Updates you can before anything else, and if you have the options, go either to Max 2013 or 2011 if this issue continues.
thanks for the help C:
As aid, its having big stacks of modifiers working on subobject selections that makes it more likely to happen. collapse the stack a bit more often and you should be able to avoid it
Also, I know the feeling, Max corrupted a file of mine several times, deleting the UV's and corruption sections of the mesh entirely. Talk about dirty code.
again, thanks for the help!
It seemed to happen when max would crash and I would start working from an autoback file. What really helped me get around it was saving incrementally outside of the autoback feature. I bound "save incremental" to ctrl-s.