Maybe you got bored and started going faster towards the end? Or maybe you started out going on instinct and then began to second-guess yourself, in a sort of "beginners luck" way?
Both sound plausible; maybe both happened, their effects on time spent per question cancelling out but their effects on accuracy adding up together.
This is a kind of "X files" thread.
Post experiences which spooked you, which made you doubt reality, mathematical or physical laws, your sanity, memory or perception. The more improbable the better, but no second-hand legends please, share only what you personally experienced. If you had the event later explained rationally please use rot13 to avoid spoilers.