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.