I was called in to consult at a site where the backups were exhibiting strange behaviour. The backup file was the normal 15Gb in size and the time stamp was from last night, but when they restored it in test there was no new data. In fact, the most recent data they could find was from a couple of weeks ago.
Looking at the DB log file, it didn't take long to ascertain that the backups were actually failing. On Windows, if the backup file already exists, probkup just writes on top of it without resetting the size to zero. Furthermore, even if no backup ever occurred, probkup still "touched" the file, changing the time stamp. The result: a full sized backup file with a valid date and time stamp but still the contents from two weeks ago.