I make some edits and press C-x C-s to save. Sometimes, out of blue emacs prompts this:

file has changed since visited or saved. Save anyway? (yes or no).

Even though I don't have the file open anywhere else, and I am sure nothing else has changed the file.

This happens rarely, only possible explanation would be that I press some combination as I bash buttons, and press save, that causes something weird. What could be the problem?