11. Galaga
Tony Stark's line about one of the S.H.I.E.L.D. agents on the Helicarrier playing Galaga was always in the script, but the later reveal showing the agent actually playing the game was a happy accident. According to Joss Whedon, Robert Downey Jr. saying "That man is playing Galaga! Thought we wouldn't notice... but we did" was supposed to stand on his own, but the director was able to make some changes after noticing what he described as a "shady-looking" extra while viewing that day's supplemental footage from the shoot. This no-doubt upstanding actor's movements meant that the game could be added in post, leading to one of the movie's funniest scenes.