I'm not sure. I don't think that the 3 that I got wrong clustered at the end and I don't think I felt any increase in uncertainty as the test progressed.
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? (if such a thing exists)
Edit: Skimmed this paper by the creators, found no mention of intentionally increasing difficulty.
(Also, my mean of 25 was from cursory skimming of this, which I found via google search and didn't check the sources. The mean found by these authors was lower, at 18 for males and 21 for females. So I'm not sure what's 'normal". Eyeballing, the distribution seems rather wide.)
Well, even if any given person had 50% probability of getting each question right, there would be a s.d. of 3 from statistical fluctuations alone.
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.