Ctrl-Alt-F1 normally means: switch to VT1. But it doesn't always
work: press Ctrl-Alt, then without releasing these keys, type F1,
then F7, then F1 again. This switches to VT1, then to X again,
but the second F1 doesn't switch to VT1: it is regarded as if
Ctrl and Alt were not pressed.

This happens both from lightdm and from my desktop after logging in.

Same problem with F2, etc.