On Linux, about twenty-five years back, on stock Red Hat Linux (and, I suspect, all Linux distributions) the bash shell used to match .* against both . -- the current directory -- and .. -- the parent directory.

This means that if you ran rm -rf .* in a directory, you'd delete all the files starting with a "." -- "hidden" files in the current directory. You'd also start recursively-deleting the contents of the parent directory.

This led to all kinds of excitement if, in a directory in your home directory, you'd try deleting all dotfiles. The rm command would also attempt to wipe out all of the contents of your home directory -- all of your files. And this isn't a system where there's some "undo delete" option, unless you had a backup system in place (which is a good idea, but wasn't something set up by default).

These days, bash doesn't do that.

EDIT: I'd also add that that was the single major reason that I initially liked zsh, a competing shell. It didn't do that by default.

EDIT2: A possibly-more-applicable-today thing I also learned the hard way: "Make backups. The cost is worth it."