What happens when you are punched in the head while casting a spell?

I am sorry if this has been asked before, or it is just really obvious in the book, but my late night search didn’t find anything. I was pretty sure there was a rule for what happened when you are casting a spell and got injured, but I cannot find it. Anyone know the answer?

Page 511. Spell Interrupt!

Usually something really bad if the spell is interpreted.

One of my favorite Burning Wheel moments was when the primarily talky character finally got an opportunity to use his B1 Brawling to do exactly this.