In 1994 I got my answer. We had installed UPS equipment on our control
computers, some PDPs and some VAXen, and that brought a bug to light that
had been well hidden before, because power outages used to kill the
whole machinery not just part. Now one of the slave PDP-11s would crash
hard after a powerout on the DEA bus (Digital Eingang-Ausgang, also known
as digital I/O, a separate module of special-purpose computers). A coroutine
added to the DRV-11 driver (after dissassembling same) solved the problem.