According to Eli Wallach, when it came time to blow up the bridge, Sergio Leone asked the Spanish Army Captain in charge to trigger the fuse, as a sign of gratitude for the Army's collaboration. They agreed to blow up the bridge when Leone gave the signal "Vai!" (Go!) over the walkie-talkie. Unfortunately, another crew member spoke on the same channel, saying the words "vai, vai!", meaning "it's okay, proceed" to a second crew member. The Captain heard this signal, thought it was for him, and blew up the bridge. Unfortunately, no cameras were running at the time. Leone was so upset that he fired the crewman, who promptly fled from the set in his car. The Captain was so sorry for what happened that he proposed to Leone that the Army would rebuild the bridge to blow it up again, with one condition: that the fired crewman be re-hired. Leone agreed, the crewman was forgiven, the bridge was rebuilt, and the scene was successfully shot.
Because writer and director Sergio Leone spoke barely any English and Eli Wallach (Tuco) spoke barely any Italian, the two communicated in French.
The skeleton found by Tuco inside the wrong coffin at SutHill cemetery was a real human skeleton. A deceased Spanish actress wrote in her will she wanted to act even after her death.
Eli Wallach was almost poisoned on the set after drinking acid used to burn the bags filled with gold coin, to make them rip open easier when struck with the spade. He didn't know the acid had been poured into a lemon soda bottle. He drank a lot of milk, and filmed the scene with a mouth full of sores.
In the original working script, Angel Eyes was named "Banjo", but is referred to as "Sentenza" (meaning "Sentence" or "Judgement") in the Italian version. Clint Eastwood came up with the name Angel Eyes on the set, for Lee Van Cleef's gaunt appearance and expert marksmanship.