While shooting the Severed Dreams episode of Babylon 5. Jerry Doyle's character was supposed to hurt his leg during the battle. Instead a mistake led to a stunt-man falling on Jerry's arm and breaking it...
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.