I had the second one not long ago. This time it is for no particular reason, so my only guess is it's a software problem. I didn't quite believe it when the panic screen appeared. (I thought it was the screen saver. That one that simulates a KP for joke. I had it but was not using it at that time.) Restarted and did the Apple Hardware Test. No problem found, and it's running smoothly since. So, I still don't know what problem it is.
Next thing is more serious. I seem to be constantly reminded of the fact that I don't have a proper backup of my Mac. I tried to imagine what would happen when the hard disk fails. It would be quite bad indeed. What I want is a hard disk that I can mirror my data to, but I don't have money to buy one.
Anyway, a few days ago my Mac was making some funny noise – symptom that there is a problem with the hard disk. So I was worried. I checked the disk with Disk Utility, and it said there was some kind of problem. Then it repaired it, and I am hoping all is well now. There still exists a need for a proper backup, though.