The Knee Defender is really just a pair of plastic nubs that attach to the seat's tray table, but they act as a brace against the chair's reclining action. The product doesn't violate any FAA safety standards, but several airlines (including United) have banned them privately. On United Flight 1462, a 48 year old man elected to use it anyway, refusing to remove the device at the flight attendant's request. The man reportedly got into a fight with the woman sitting in front of him: words were said, cups of water were thrown and the plane eventually landed early in Chicago to evict both of the unruly passengers.
For what it's worth, the Knee Defender website specifically advises removing the devices at the request of airline personnel, and throwing courtesy beverages at another passenger is never appropriate behavior. Still, it's easy to sympathize with both passengers: nobody wants to relinquish personal space on a cramped flight, but nothing's worse than trying to use a laptop while the person in front of you is reclining. On the other hand, both passengers were enjoying the extra four inches afforded by economy plus -- so next time you fly, maybe just don't be a jerk, regardless of which side of the tray table you're on.
[Image credit: Alamy]