A merge of <File> produced a 0 byte file. Data may have been lost. Would you like to undo the merge? (recommended)

This error has the following cause and solution:

• A zero length file indicates a possible internal VSS error resulting in loss of data during the merge operation.

  Recommended:  undo the merge by clicking Yes.  If you are certain, however, that you've performed an action that resulted in a valid zero length file, you can click No to save the zero length merge file.