Better than throwing a rod, but still very bad.
The race you're about to see starts off sounding fantastic. It's a 2013 Shelby GT500 with a new pulley, engine tune and a complete intake and exhaust, up against a C5 Z06 Corvette with a cam, full intake and exhaust and a dry nitrous kit. That's the kind of street race we want to see, and indeed it starts out just as great as we had hoped. But then the Mustang suddenly drops back, and we see that there's a lot of smoke coming out of the exhaust.
This gives every appearance of being a blown head gasket, and while there aren't any good kinds of engine failure, they do still get much worse than this.