In the end, the conversation erupting around the slight alteration of Tony Stark’s facial hair actually serves to emphasize how well Anthony and Joe Russo executed the first half of the massive Marvel Cinematic Universe project that is the final showdown with Thanos. “Infinity War” faithfully and effectively delivers on the multiyear buildup that leads to Thanos’ arrival, pitting a crowd of vigilante good guys against a stop-at-nothing zealot and leaving viewers gutted by one of the most dramatic closing sequences in cinematic history.
As u/Motor-Anteater-8965 conceded in a follow-up comment, chances are, the mustache error in question is a result of having reshot the scene weeks or even months apart, with other users pointing out that Peter Parker’s (Tom Holland) hair changes on Ebony Maw’s (Tom Vaughan-Lawlor) ship too.
Redditor u/DefNotAShark concluded, “Doesn’t really take me out of the movie (especially this movie) but it’s fun to spot.” It’s a fair observation. These kinds of minor changes only reinforce how well certain films pull viewers into the suspension of disbelief — a state of mind that is shattered when that extra mustache hair becomes impossible to ignore. For now, we’re just going to go with u/BattlingMink28’s in-story explanation: “He was just stressed.”