Out of disk space
One use that I make of SuperDuper! is to maintain an exact clone of my laptop's internal drive. The clone is the same size, because it's an identical model disk. In the event of a disk failure, I find a screwdriver and I have my laptop back in service in ten minutes.
I can't believe that I'm still regularly getting "out of disk space" errors, cloning my user partition. Usually, I have plenty of working space on my system partition, but I have 10 GB of media files that change from time to time, and less free space, on my user partition. SuperDuper! tries to write the new files before erasing the old files, and doesn't have a plan B.
If "Disk Inventory X" can find all the large files on my disk in under a minute, why can't SuperDuper! check to see which large files will be deleted, and delete them in advance? This would avoid the problem with high probability.
Alternately, if the code under the hood is sufficiently modular, it would be trivial to write a fallback "Plan B" where when SuperDuper! encounters this error, it starts over only deleted, then starts over again in normal mode.
Generally, when I get this error, SuperDuper! had plenty of time while I was gone for three such passes, but it froze like a deer in the headlights. It's times like this that I remember wanting to write a similar program as freeware.