5. Viggo Mortensen Breaks Two Toes - The Lord Of The Rings: The Two Towers
In The Lord of the Rings: The Two Towers, there's a memorable scene where Aragorn (Viggo Mortensen), Legolas (Orlando Bloom) and Gimli (John Rhys-Davies) stumble across what they believe to be the burned corpses of Merry (Dominic Monaghan) and Pippin (Billy Boyd).
In his rage, Aragon kicks an Orc helmet, which Mortensen didn't realise was an actual steel helmet rather than a lighter prop, causing him to break two toes and consequently scream in agonising pain while dropping to his knees.
The Awesome Acting
At first, director Peter Jackson didn't even realise this was a gaffe and simply believed that Mortensen, brilliant actor as he is, was tapping into something deeply primal and painful inside himself.
Upon realising the truth, however, Jackson decided to keep the take and use it in the final film, impressed that Mortensen continued to perform the scene without missing a beat.