Yul Brenner and Steve McQueen had clashing egos on the set of The Magnificent Seven. Brenner would make piles of dirt to stand on to appear equal to McQueen who was 2" taller. McQueen would respond by pulling focus during Brenner's lines by adjusting his hat. McQueen was not invited to the sequel.
We all know the "Viggo broke his toe" story but what's another movie fact you're personally compelled to bring up every time without prompting? For me, it's the fact that Robin Tunney is wearing a wig in The Craft because of having just shaved her head to save her job filming Empire Records.