Back in the BBS/FidoNet days someone sent me a comic GIF that, when renamed with a .COM extension, would execute under DOS. It was a nifty little demo, although .COM files were going the way of the dinosaur around then.
The characters you were typing are (probably) the code for a jump to an entry point somewhere else in the file.
I think what parent is remembering were characters that effectively created a jump instruction at the beginning of the file.
[1] https://en.wikipedia.org/wiki/DOS_MZ_executable
edit: [2] Section 6 of the paper talks about that, just noticed.