I've never seen this behavior either, and I use TextMate pretty much constantly. Forgive any dumb questions, I have my troubleshooting hat on...
* Are you saving files to the local disk, or a network share? * If local, is anything else accessing the file? (ie., is it open in another editor at the same time?) * If network, what's the filesystem and how is it mounted? * Do you have atomic saves enabled? * Does this happen when you add lines to a file, or only when you delete lines? * When you see the garbage, is it text from the same file or from a different file? * If you save the file (but not close it in TextMate) then immediately 'cat' it from the command line, is the garbage there? * What is the file's text encoding? Do you have any Unicode characters in there? * Do you have any TextMate plugins installed?