Comment 2 for bug 1350580

I fixed the problem.
Looks like I am pressing Shift button for time more than 10 seconds, when I thinking.
Slow Keys are enabled after such type of Shift pressing.
Accessibility and Show Keys are denied in my system, but because of bugs in some desktop environments (such as xfce4), these settings are ignored.
So solution is to add such script in autostart after graphical login to Xfce:

 xkbset -sl # Disable slow keys (if already active).
 xkbset -a # Disable accessibility to prevent it from switching on.

Don't forget to install xkbset before this.

Related bugs with this problem, where you can found discussion of the problem:
https://bugs.launchpad.net/ubuntu/+bug/1002951
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=677173
https://bugzilla.redhat.com/show_bug.cgi?id=816764

To detect problem existance in your system dissable Accessibility in gui setting and then run:
xkbset q | grep "Accessibility Features"

Solution is from here: https://www.unix-ag.uni-kl.de/~guenther/slow-keys.html
If "On" then you have the problem.