Some of those fail.

As for the rest, you're mistaking the point. Either they do it or they don't. If they do it, it works. If they don't, it doesn't. If a mage casts a spell or a communer does a commune, either they get it right and it then goes as it always has, or, as I'm suggesting, it has a small chance of just not getting off in the first place, for whatever ic reason anyone wants to include. A missed word, a bug flying by and distracting, whatever.