Sometimes a player will leave the game for whatever reason for a LONG time, and so you get used to the idea that you're only dealing with an AI robot when it comes to that player, and that changes the way you play - you may not attack that player so vigorously because you can generally be less concerned by an AI player's capabilities.
Then suddenly - sometimes a long, long time later - the person will return to the game and may find that, no thanks to their own efforts, they're in a hugely advantageous position, and you're not prepared to suddenly be dealing with a human player again.
It's really not fair when that happens - even when I've (inadvertently) benefited from it once or twice, I knew it was an unearned and sneaky sort of victory. So maybe there should be a time limit on how long someone can be gone from the game and still come back - like 6 turns, or maybe 12, or something. There's really no reason not to.