This error has the following cause and solution:
Click Yes, and VSS checks it in anyway. (Clicking Yes All checks in all selected files.) A checkin creates a new version of the file, with an optional comment.
Click No, and VSS undoes the check out of the file. (Clicking No All undoes the check out for all selected files.) No new version of the file is created in this case.
Note To avoid this message in the future, disable this notification on the General tab that is accessed by selecting Options from the Tools menu.