I was trying to make a call using the screen dialing pad, and after every digit I typed, it would get erased, as if I had pressed the backspace key withing half a second. Consequently I never got past the first digit. I fixed it by opening the physical keyboard and typing there.

This has happened only once in the 5 years I have been using the N900; I don't use the dialing pad that often, though. Either way, not comforting.

Did it occur again after a reboot? Any strange processes seen from terminal?

Has not occurred since. I did not check for strange processes then. I can check now but I hadn't installed anything new.

I had the same, and recovered a stable backup, never figured out what happened. felt creepy indeed, the typings disseapering.