Not everyone makes the best moves all the time and occasionally it costs them the game.
St Amant misses 28...Nb6!!
Kieseritzky vs Saint Amant, 1840 (C00) French Defense, 39 moves, 0-1
Morphy misses 18.Qxg7!!
Morphy vs T Lichtenhein, 1857 (C36) King's Gambit Accepted, Abbazia Defense, 28 moves, 1-0