Bride wasn't happy with her husband-to-be's vows, so she requested a 'time out' in order for him to go to the back room and come up with better ones. When the guy marrying them politely explained that he was on a very tight schedule, she called him a 'lying bastard' and huffed back up the aisle into the room. We kept everyone else seated, and she appeared a few minutes later with freshly written vows for her future husband. He read them, and the wedding continued as planned. Again, uncomfortable.
It's rare that we get a groomzilla, but one in particular stands out. The groom wasn't happy with anything. The bride had her heart set on white roses, but he said he didn't like roses, so he requested sunflowers and white chrysanthemums. When we presented him with an example display of sunflowers and chrysanthemums, he said he didn't like that either and requested another two or three separate types before finally settling on a bunch of plain peonies. He did the same sort of thing with the food, music, entertainment, ignoring the fact that we have deadlines to meet. The bride had zero say and didn't try to fight him on it.