If I change one character in an open, previously saved TM document, switch away from TM to the Finder, and then switch back to TM, I get this warning:
"The file for the document at ~/Desktop/yourfile.rb has been modified by another application. There are also unsaved changes in TextMate. Do you want to keep the TextMate version or revert to the version on disk?"
This warning is well-intentioned but (under these circumstances) false. m.