Man forcibly removed from overbooked United Airlines flight

This is bad planning on the airline’s fault. Why wasn’t the mistake spotted before boarding? I thought computers weren’t allowed to make these sorts of errors. Were two seats allocated at the same time? Perhaps people were on the phone and the call operator didn’t wait for confirmation that the booking was complete. This could have been sorted without all the hassle on the plane. They might have found someone who’s willing to transfer to a later flight. Instead, they decided to make a decision against someone’s will which caused more conflict that they’d thought. Was he the last single passenger to buy a ticket, or did they not like the look of him or his profile? I wonder how late the flight was and what the man’s reaction was. It doesn’t sound that he’s happy by the outcome. Would he get arrested for something that wasn’t his fault? These companies need to wake up otherwise people will try and avoid them.