When you unshelve a shelveset, the following dialog box may pop up:

Cause:

The item exists in your pending changes window. It might be caused by that the "Preserve Pending Changes Locally" option was checked when you performed Shelve.

Resolution:

Undo the corresponding pending changes. Or, if you do not need to further edit the files in the shelveset on performing Shelve, please uncheck the "Preserve Pending Changes Locally" option.