So the adoption group I work with contacted the shelter for an explanation. This is the response I received from them:

"> What happened with Peanut was simple human error. Peanut and another cat at the shelter resembled one another very closely in appearance. It was the other cat that was on the euth list that day. The kennel manager didn't cross-check the paperwork carefully enough and that's how Peanut wound up being PTS. I know the kennel manager as well - and she's beating herself up for what was a big, grave, tragic mistake - and one unfortunately that could have been prevented as Peanut's paperwork was in order, correct, and aligned with his control number.
>
> The shelter has had a series of meetings to discuss this particular issue. The goal, obviously, is that it NOT happen again and that the shelter institute additional checks and verifications when any cat is scheduled for the euth list."