The fight started on a United Airlines flight because one passenger was using the Knee Defender, a $21.95 gadget that attaches to a passenger's tray table and prevents the person sitting front of him from reclining.

The dispute on United Flight 1462 from Newark, New Jersey to Denver, Colorado, escalated to the point where the airline decided to divert to Chicago's O'Hare International Airport, according to Transportation Security Administration spokesman Ross Feinstein.

A man in the middle seat of row 12 was using the Knee Defender to stop the woman in front of him from reclining while he was on his laptop. A flight attendant asked the man to remove the device, and he refused. The woman in front of him then stood up, turned around, and threw a cup of water at him, according to a law enforcement official with knowledge of the situation who spoke on condition of anonymity because they are not authorized to speak. That's when the United flight decided to land in Chicago; the two passengers were not allowed to continue to Denver.

The Federal Aviation Administration leaves it up to individual airlines to set rules about the Knee Defender device. United Airlines said it prohibits its use, like all major U.S. airlines. Spirit Airlines and Allegiant Air take the reclining mechanisms out of their seats, leaving them permanently upright.

Chicago Police and TSA officers met the flight when it landed in Chicago, spoke to the passengers — the man and the woman, both 48 — and "deemed it a customer service issue," Feinstein said. The TSA would not name the passengers.

The plane then continued to Denver without them, arriving one hour and 38 minutes late, according to the airline's website.

The Federal Aviation Administration can impose a civil fine of up to $25,000 for passengers who are unruly. In this case, no arrest was made, according to airport spokesman Gregg Cunningham.

The Knee Defender device prevents the passenger in front of you from reclining their seat. It is prohibited on most airlines. Image: Knee Defender

Both passengers were sitting in United's Economy Plus section, the part of the plane that has four more inches of legroom than the rest of coach.

Passengers usually pay an extra fee for a seat in the section.

Additional reporting by Mashable