This came up a couple of weeks ago. I had been DM'ing my regular group for 2-3 sessions without the group's only female player because she was rehearsing her part for a play she was going to be in. So, I promised to play her elf wizard for her since none of the other players felt like they could handle two characters at a time.
So far so good, but I end up forgetting to play her character for those sessions and forget giving her half the XP that the other characters get (that's a houserule of mine; if you can't make it for a session, you're awarded only half the XP). Then all of the sudden she writes to me and tells me that if I don't give her a certain amount of XP or gold, she'll quit the game. This ultimatum came out of thin air, no warnings, no "hey, I think you forgot to give me XP for the last couple of sessions", just a plain "give me XP or I leave!"