> It’s quite impossible.

Do you know all about 1970s operating systems? Why do you find it hard to believe that a loop deleting backwards from the buffer wouldn't, say, zero out the stack frame and then a function return crashes it?

> Was it identified as a bug or vulnerability?

What difference does that make? It crashed the -10. It was reported.

> What OS, even?

The TOPS-10 operating system, or its predecessor. I don't recall just when TOPS-10 came out. It was 50 years ago.