The latest version of xdm tries repeatedly to start X when the X configuration is borked. So when I install a new linux kernel and reboot, my missing nvidia blob causes the screen to switch from the console I'm on to the F7 console over and over (dozens of times per second). The main symptom istThe screen flickers rapidly.
Attempts to login when the PC is in this state are foiled because the keys I strike might come out on the F7 console, so entering root password successfully is nearly impossible.
I think XDM needs to die after one failed attempt to launch X.