Something, that some window manager already could do was now implemented into the X-Server: Hold a key for more than 15 seconds to activate a key hold, so that every further key must also be pressed for 15 seconds. That behaviour is called sticky keys and it is annoying, especially when working with image manipulation programs (in which long key presses use to have a special meaning).

It is not possible to deactivate this problematic feature in the default configuration file xorg.conf yet, but there is xkbset to the rescue:

xkbset -accessx -sticky -twokey -latchlock

And this command can be added to the ~/.Xresouces to survive a restart. Create this file if it does not exist.