In Meat Your Maker (season 1), Mordo and Riggs manage to put the blame for the sausage fiasco on Benson and get away with burning them. They screw Benson over so badly I used to be pissed with this episode and consider it the worst of all.
Much later (season 3) came Cool Bikes and Muscle Mentor, and in these Benson is a jerk to both and to Rigby, respectively.
It took me long after watching these three to realize the "cosmic balance" they establish. The show would be pretty annoying if the same character always got it their way (I'm looking at you, god awful Tom & Jerry Kids), so it's very relieving that injustice keeps changing its target. Nowadays I'm in peace with Meat Your Maker. XD
Can you people point out other "unfair" episodes?