See Also
This dialog box appears when the items you want to undo check out have been changed. You can set whether to show this dialog box on the Tools->Options->Files Tab.

Remarks
This warning has the following causes and solutions:
1. You are about to undo the check out of a file. If you do so, you discard
all the changes made since you checked out the file.
2. If you want to put your changes into VSS instead of losing them, use the
Check-In command instead.