Emacs has a tabs-to-spaces feature when you hit the tab key. But Makefiles require actual tabs not any whitespace (p.s. NEVER do this!).

So to get a tab, I can't type it. I have to copy-paste it from somewhere else. (Ok I could actually toggle the spaces-to-tabs feature but that requires remembering some "incantation".)

Result: all my Makefile tabs are descendants of a tab in a Makefile I was banging my head on, trying to figure out WTF was wrong, in a CS assignment at Yale in 1994.