@simon_brooke I think it's clear from context that "high chance of making your whole machine unusable" refers specifically to XOrg/XFree86 running as root with its own video drivers directly poking hardware, not any conceivable X11 implementation.

On platforms where X wouldn't crash there isn't any evidence of what would have happened if it did.

On Linux my experience doesn't agree with yours and I have frequently had to power cycle a machine or best case ssh into it and reboot it because XFree86 had locked up all human I/O interfaces.

@marcan