What corrupted my *.htm files?

RoboHelp 8.0.0.203 would crash every time we tried to open a certain project.

I looked at the project files in RoboSource Control 3.1, and determined that the *.htm and *.css files were not "viewable", as RoboSource Control insisted that they were binary instead of text.

I checked out the affected files (~3,000 of them!) and determined that the problem was 3 garbage characters at the very beginning of each file. Most of them had

^@^@^@

but a few had other junk, but always 3 characters.

I stripped out the 3 trash characters from the beginning of the affected files, then checked them back in to RoboSource Control. Now we can once again open the project in RoboHelp.

Anybody have a clue what could have caused this corruption?