[alecst]

I guess... I mean, it is more mysterious than you suggest, given that this fellow had been foraging for 10 years apparently.

It's like a driver mixing up the gas and the brake pedal. I don't really have an explanation.

[floatingatoll]

Ah! That sounds like a job for an NTSB-type analysis, because most human WTFs are built on a series of improbable events.

Maybe their glasses were fogged? Maybe the afternoon light was at the right angle to hide the 'gills' enough that their check failed? Maybe they had a time-efficient shortcut that finally found an edge case they hadn't planned for?