Thanks for these responses. I can see now why players & GMs probably never bother with magic failure rules. The mechanical systems all seem fiddly. Dungeon World's narrative approach looks like the best way to fold it into the general gameplay without being a nuisance.
I'm looking for an examples of explicit game mechanics for spell-casting failure and consequences. Dungeon Crawl Classics is the only one I've seen; it has pretty dire outcomes. D&D 5E (and previous editions) assume if you know the spell, it always works perfectly. I'm curious about other...