I've only ever had explosives get thrown once in my campaign, and when it happened, the initial roll to place it correctly was a success... so I didn't notice until today how weird the rules get if you miss.
On any other failure, it lands somewhere else. All relevant characters (as determined by the GM; generally those in the proximity of either you or your intended target) must make a Fortune Check at Difficulty 12. The character with the lowest failed check is at ground zero of the explosion.
See the problem? GMCs don't have Fortune ratings. So if a grenade misses, it's almost always going to land at the feet of a GMC. For it to bounce to a PC, that PC would have to get a Way-Awful Failure that is also lower than all the GMCs rolled, and their rolls started 6 to 8 points below yours. That's going to be pretty rare, especially since you were probably throwing that grenade at a group of a dozen mooks.
Which is... fine, I guess? It certainly fits the genre, and players are still kind of in danger if they were punching the GMC who the grenade landed near...
But it's also really clunky, and could easily involve a dozen die rolls (one per character) to resolve this single event, in a game that's otherwise for the most part pretty fast and simple.
Also, the Wild Grenade schtick seems actively bad now that I understand this. Chances were already pretty good that you'd never get hurt by your own grenades if you don't have that Schtick. The schtick is effective "take 20 damage and pay 1 fortune to turn your area-effect weapon into a single-target weapon that can't target the person you most wanted to hurt". That seems awful.
So I'd be interested in house-rules to make this situation faster to resolve, make the schtick a better choice as well. If anyone has any cool ideas, please post them here. (Heck, I'd also listen to arguments of why I'm wrong, since I've never really seen it play out as written.)