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? Mayb

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?