Virtual terminal freezes

[cas]

Hello,
it happened several times to me, that leaving a virtual terminal via "ALT+LeftArrow" left the virtual terminal freezed. (Astonishingly this isn't always the case.) I cannot say when or  why this happens or not.

No key helped. Neither Alt-SysRq (but the latter doesn't seem to be activated anyway).
I had to do a hard reboot each time.

C