A call to the eSage helpdesk from one frustrated user was quickly resolved. The user couldn't log in to their computer. Preliminary questions ruled out they were punching the wrong keys for the password, so I was a bit mystified and hoped it wasn't going to be a a difficult task involving booting into recovery mode etc.
Fortunately, I had direct access to their Mac remotely so could see their login screen.
I could then see the caps lock key was turned on. Once I explained the on-screen symbol for this and what the key does, problem solved!