I've seen more than one half-joke-half-serious chunk of code that would "encode" arbitrary info into stack traces simply by recursively calling `fn_a`, then `fn_s`, `fn_d`, and `fn_f` before continuing with the actual intended call, giving you a stack trace with (effectively) "asdf" in it.

They've also been useful more than once, e.g. you can do that to know what iteration of a loop failed. There are of course other ways to do this, but it's hard to beat "stupid, simple, and works everywhere" when normal options (e.g. logs) stop working.