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If we're allowing supernatural explanations, maybe they were subjected to Laser-Guided Amnesia or what they saw was part of an Extra-Strength Masquerade and triggered their Weirdness Censor (they saw the thing, but believed it was something else). Usually the work will eventually reveal this so we needn't have to guess.
It's also possible they Failed a Spot Check and really failed to see something right in front of them.
Edited by KD
I'll check those out. Thanks!
Someone says that something revealed before him is completely unknown to him and a complete surprise, and he isn't lying, despite the fact that he was around when that something occurred and it would be impossible for him to be ignorant about it.