On Friday I was trying to get a set of .dwg files out (it didn't happen). I would export the first few files fine from Revit, but then suddenly all my previously exported drawings would get the drawing view of the next exported drawing inserted into all their viewports!

Luckily, after some head scratching Tech Support figured it out. What was happening was that the way the file name appears when it's exported starts the same for all drawings "Project name-rvt-drawing sheet-....). The problem with this format is that if the Project Name is long the first 35-45 characters of each file is the same so the computer stops distinguishing between files. So if any subsequent file is modified the computer will ovewrite all the previous filess with the modified file because it can't tell the difference.

So if you have a somewhat long Project Name don't use the default export name. This wouldn't be a problem if the default name had the sheet number and title at the beginning instead of the end because then every file name would start with different characters.