MUMPS (aka 'M', MUMPS loves brevity to the point of obscurity) was - without anything else coming close - the weirdest language/runtime I've ever had to work with. Fun times, though, especially when someone accidentally deleted the editor. Now, how that could happen requires some insight into how mumps works under the hood: the in system editor (which, on that particular distribution was called 'e') is a string. Just like any other string you can assign a value to it and there weren't that many safeguards in place to protect important strings.

Recovering from that particular 'oopsie' cost me a day and a bit, part of which was to write a rudimentary editor, without an editor. I think I got some apple pie out of that one :) I don't regret having seen the last of 'MUMPS' but it is very persistent and in healthcare and insurance you still come across it every now and then.

Mumps has some interesting aspects though, one of these is that it raises evaluation of expressions to a way of life, and in that sense it is closer to a functional language (but with statements, if that makes any sense) than your typical interpreted language from that era, which had strict barriers between code and data.