Probably going to twist off a lug bolt head...
 

On my 99 e300. Apparently the person before me cranked one particular one down as tight as the impact would go. The head is already angling up as I was trying to break it lose with a breaker bar. If the head twists off, will I still be able to get the wheel off (the other 4 lugs are fine) or is there something in the design of this lug stud that's going to cause it to stick? I have a magnetic induction heater if I can just get the wheel off. I'm guessing this is probably best left to a shop that can just weld a nut in place on the end and try to get it out that way?

This happened to me once when I used a too-long bar to torque my lug bolts and went too far.

You won't be able to get the wheel off but the solution is simple and quick:

1.) Get an Easy-Out bolt remover kit
2.) Get a cobalt or black oxide drill bit

If/when the head twists off, drill a hole in the remnant of the bolt and tap the Easy-Out into it. Then use your breaker bar and appropriately-sized socket to crank out the bolt.

Slide a "cheater bar" over your breaker bar for extra leverage. While applying steady pressure to loosen the bolt, have a helper strike the back of the breaker bar, driving the socket into the bolt hole. I've never had this method fail and I've done it a lot.

I saw several which had the head twisted in half and left the shoulder bonded to the wheel. Saw two of those in Greer LKQ yesterday.