[math-fun] Miscounting errors

Anyone have a favorite puzzle in which miscounting plays a role? My favorite is the classic bookworm puzzle (see https://math.stackexchange.com/questions/1271651/how-is-this-true-bookworm-p... ). Another example: A man was born in 50 BC and died on the same day in 50 AD. How old was he when he died? I'm especially interested in puzzles that lend themselves to solvers committing fencepost errors, and off-by-one errors more generally. Jim Propp