I've only ever seen a real spontaneous fire in a
monitor though, not in
a system unit. It's probably possible if there's enough dust and
something gets hot enough though.
I've had an RK05, or more precisely the blower fan, catch fire. Being a
good, solid, DEC device it carried on working with flames visible through
the front window (!). I retraced the heads, shut the system down, and
fortunately the flames went out. It turned out to be insulation breakdown
in the fan motor windings.
A friend of mine gave me anothe defective RK05 blowwer, this one with bad
bearings, and using parts of each I got the drive running again.
-tony