There have been no backup errors.
Recently I had to do a restore of the VM. All seemed fine. The size was right. When I ran it, however, the VM was corrupt.
The VM is made up of many large files. I think Retrospect backed some up at different times, depending on metadata changes, file locked, etc. Normally that would be fine, but they all had to be the same version for the VM to work.
Fortunately I had a completely separate manual all-at-once backup done when VMWare was turned off. It was a few days old, but we didn't lose anything significant. Sometimes paranoia is a good strategy.
A cautionary tale! Virtual machine backup is tricky.