Now that's scary. Let's just call it "Chaos Bomb" and have everyone in the area mishap just because

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To trigger a one-time mishap ala Minor Mishap, maybe one or two levels less? If Major mishaps are 4th-7th, 3rd level spell Major Mishap. For a Wild mishap, which is normally 8th and 9th level spells, a 7th level spell slot seems a bit much. 5th? Too weak, since some of those effects are very powerful, but it
seems such a waste of a 6th level slot, even though that's where I think it should go.
Some modifications for the higher level mishap spells to make them useful could be as you said, multiple mishaps. Or one mishap per round for X rounds, centered on a person or object (saving throw). One mishap
per target in an area (yeowch)...nasty things like that.
I'm contemplating making one spell per spell level of this nature, and a feat to allow a wild spellcaster to spontaneously convert a spell of the appropriate level to a mishap spell of the same level. Will post (more complete) ideas in the next day or two.