A couple days ago I was really captivated solving chess puzzles on lichess and I encountered a particular puzzle that didn't make sense to me (my puzzle rating is ~1800). Then I realized it's white to move, and I've been pondering on black moves for the last 10 minutes... Mistakes a human mind can make while playing chess really surprise me, even while doing another intellectual activity like programming, learning math, reading a book etc you don't realize how notoriously prone your brain to errors until you have an objective measure to compare your reasoning to. Fascinating.