With all my flying, I've never had this happen. I was on Southwest and they had the wrong series number for the plane in the system so they let on a bunch of standby people and didn't have seats for them.
They made all of the standby people get off then they seated them first come, first served. At this point, there were only scattered seats left. We were in the third row (and there were three of us) but the middle seats were open in row two on both sides. A guy and his daughter (probably about six years old) got on and he told her they couldn't sit together and asked her to get in one of the middle seats.
She flat out refused. You could tell she was afraid to not sit right next to her Dad. He tried to cajole her to no avail then asked the two guys on the end of the rows if one of them would mind sitting in a middle seat so they could sit together.
They both flat out refused. A couple a few rows behind us offered to split up and gave their adjoining seats up. Nice.
OK, I know they bordered early so they could get the primo seats but the flight was just from Phoenix to El Paso. One hour long. Come on, guys, you couldn't suck it up for one hour to make a little girl feel comfortable on a plane?
They weren't even embarrassed by it, either. I would have felt like a total loser if I'd done that.
It was one time when I wished I really believed in Karma because theirs took a big hit.