Sounds like the file is in use by something? Probably the simplest, but definitely not the most efficient method - it should work after a reboot.
Alternatively, check you don't have it open in any of your terminals/editors - Though I'm guessing if you have that file open by hand you SHOULD know this.
Check what's running at the moment, if there are any editors hanging, kill them. Anything outside of that, I'd be looking at security issues.