Loved that RSTS, BTW! The error message texts were in a file and I always wanted to change "UNIBUS SYSTEM TIMEOUT FAILURE" (a hardware error) to "IMPENDING SYSTEM CRASH AT LINE xxx".
On topic, here's one you see very often:
Error: "The operation completed successfully."
It sounds like an oxymoron until you realize that in many systems, error code 0 means "successful completion". Your code, to quote in LOLspeak, made you an error but it eated it.
(Back to RSTS: Error 0 indicated successful completion. It also pointed to the banner text of the system, something like, "RSTS/E V06C Salem Schools PDP-11/60". This could be REALLY confusing to encounter in a program!)
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