I don't care when someone is preferred, but usually it sucks when (generally, or because someone else is preferred) someone is treated worse for no reason.
But, the only time I've seen that since school, is in WoW. The girlfriend of an officer of us was preferred by him over anyone else when it came to giving raidspots / loot and so on. Being the guild and raid leader I intervened, but obviously too late, since the drama already started.
And in school: We had some teachers extremely preferring some students. I specifically remember one I had from 6. to 8. grade, which had in each class one person which she preferred over anyone else. Those were generally not even the best students of their class, which usually get preferred by teachers. My brother had the same teacher too and had the luck of being "the one" in his class.