How about multi-boxing for crafting? Macro your craft and press on #1, press on #2, press on #3, press on #4...make 4x the profit in 1x the time...
That kind of abuse.
In Atlantica Online, people openly abused special instances called individual dungeons (from 1 to 3 players allowed). They need a key and stay open for a set duration, under which they have to be completed to get the bonus reward. Everyone in the instance gets the bonus reward. You go solo, you get 1, you go with 3 total, each gets 1. And those instances didn't level sync you, so you could overgear most of them, by far - and thus solo them. Then bring your 2 multibox, and get triple reward. Thus making the honest solo-boxer get 1/3 of the reward.
How did it affect the solo-boxer? Well, the best rewards were sellable (new mercenaries sometimes), for billions. So it gave a much higher supply, reduced the price faster and also caused inflation vs that player because every multiboxer would make more money/time.
Legal, perhaps. Cheating, definitely.
For dungeons, well there's tomes. She gets 300 philosophy tomes per run, you and I get 100 tomes. How is that fair?