RT-11 on a PDP-11/23 was where I came to once in the world of computers.
Later on there appeared Soviet clones of same but their MTBF (a.k.a.
mean
time
between
failures) was somewhere on the order of 3 to 5 hours or so only, mainly due to the monitor static electricity breakdowns that used to also kill the CPU video controller:
Intel PC's and compatibles were unavailable on our side of the Iron Curtain until the late Gorby times due to the COCOM restrictions.