Time Machine Silently Excludes Critical Data
Status 2013-01-19: extant in OS X 10.8.2, probably earlier OS versions also
Time Machine silently excludes critical data volumes from backup, leaving data unprotected, which means that weeks or months can go by without detection or protection.
As shown below, I was lucky— there was nothing left to backup. But sometimes there is, so the user gets no warning clue at all, unless one is “lucky” enough to have no volumes left to be backed up— then this dialog appears, which is a warning clue.