Saw a guy in the history section methodically rubbing every single book spine with a lemon wedge. He had a whole bag of them and was leaving these little wet, sour streaks on the bindings. Didn't say a word, just kept on wiping.
That lemon wedge thing reminds me of my friend who saw a woman at the library whispering numbers into the return slot. She was just counting out loud, like "thirty-seven, thirty-eight," real quiet but super focused on it. My friend said it was the most calm, weird thing she ever saw.
That lemon thing robinf51 mentioned is nothing. I once saw a guy at the library using a slice of bologna as a bookmark, just left it sticking out of a cookbook.