Special keys -> key name instead of action

[tDeContes]

Sometimes (not very often), when I tap special keys (like arrows), it writes the key name (e.g. ArrowRight) instead of doing the corresponding action.
If I see it I can undo it quickly, but it's annoying anyway.

Do you know what could happen at these times?
I'm not sure whether the source of the pb is Text-Insert or Thunderbird.