In my case I had rounded off the bolt head in a few spots with an open ended wrench after a few tries not realizing it was going to be stubborn. Had a hard time getting the correct sized close ended wrench on with the warped sides. Went up a size by switching to metric and getting a decently good grip. That stripped it further, but warped the head even more so I was able to get the right sized closed wrench on it with a few "gentle" taps of a hammer. It juuuuuuust barely came off. The head was mutilated to hell.

I've seen people use a grinder to grind down the sides of the head and use a smaller wrench to try again. Or cut a plus in the head and try a screwdriver. But that's assuming the head hasn't popped off yet.