Recently I have been leaning a lot on my shift keys, it seems.
This turns on Slow Keys on my machine, which is exceedingly annoying - XFCE doesn't even acknowledge that they keyboard shortcut for activating this, probably very important if you need it, functionality has been hit.
The sequence roughly is:
- "Uh oh, my keyboard doesn't work?!?"
- See the notification about slow keys
- Go to the Settings → Accessibility menu, activate Slow keys, then deactivate Slow Keys
If you search for this problem, you will find that people have been complaining and discussing it for years and years.
Here is the pragmatic solution:
apt-get install xkbset + add
xkbset -accessx to your startup (session, script, whatever).