As with any film, whether it's an action film, a period drama, a comedy, whatever - injuries will happen either through accident, over-applying force or just through sheer bad luck.
There's always something of a grey area as to whether or not these scenes should make it into the film, but sometimes it can mean a much more convincing performance when the actor is in real agony from their injury.
Here's a few examples of some particularly well-known scenes - and the injuries that made them.
8. DJANGO UNCHAINED - Leonardo DiCaprio cut his hand smashing a glass
Probably one of the most recent and well-documented on-set injuries, Leonardo DiCaprio's crowning moment in Django Unchained is when he discovers Christoph Waltz and Jamie Foxx aren't who they say they are and smashes a glass in exclamation. As it turns out, the glass was real, the blood was real, Quentin Tarantino didn't want to yell cut and ruin the moment and everything that happened after that was more or less unscripted. For example, Kerry Washington was visibly shocked when DiCaprio smeared blood over her face, you can see the actor to DiCaprio's right jump when the glass shatters and the glass he picked out his hand afterwards was actual, real glass.
7. BLADE RUNNER - Daryl Hannah chips her elbow
In an early scene in the film, Daryl Hannah's character, Pris, runs away from someone as they approach. On set, director Ridley Scott had wetted the ground so that the light would bounce off and create the noir effect he was looking for. Trouble is, when you're running in six-inch heels, it gets hard to stop. As you'll see below, Daryl Hannah crashes into a parked car - which wasn't planned - and cracks the window with her elbow. Again, that wasn't planned - and neither was the chipped elbow and resulting scar she got from it either.
6. NIGHTCRAWLER - Jake Gyllenhaal got 40+ stitches (and seven years' bad luck) from breaking a mirror
Jake Gyllenhaal's performance in Nightcrawler really was incredible and the way in which he approached the character - by making him so reserved and cold - made it unsettling when he eventually snaps in a pivotal scene. Gyllenhaal went hell for leather and slammed the mirror cabinet shut and the resulting shards sliced his hand open, resulting in 40-odd stitches in his hand. That take made it into the film, but Gyllenhaal didn't even flinch - although you do see him looking down at his hand when it cut open.
5. THE LAST SAMURAI - Random extra gets hit in the balls by a horse
It's a blink-and-you'll-miss-it moment, but imagine getting belted in the nutsack by a huge stallion that has Tom Cruise on top of it. Tell that story in a bar and you'd never have to buy a drink again.
4. GOTHIKA - Halle Berry's arm was broken by Robert Downey, Jr.
Although it's not a terribly well-known film (or particularly good, for that matter), Gothika does feature some pretty brutal physical moments in them - one of which caused filming to shut down eight weeks. The scene, which made it into the film, involved Robert Downey, Jr.'s character physically restraining Halle Berry from injuring herself in a psychic ward. The take - which made its way into the film - saw Downey, Jr. slam Berry's arm down. The resulting force actually popped Berry's arm out and Downey, Jr. quickly backed away.
3. THE GODFATHER - James Caan broke a guy's ribs
In the famous scene where Sonny Corleone arrives at the home of his brother-in-law and beats twelve shades of shit out of him, actor James Caan was really going hell for leather. In fact, when he hurled Gianni Rossi over the railing and started hitting him with jabs to the ribs, he wasn't holding his punches. The scene was shot in one take, and Rossi ended up with two broken ribs for his trouble.
2. TERMINATOR 2: JUDGEMENT DAY - Linda Hamilton had permanent hearing damage from the elevator scene
In Terminator 2, there's a scene when Linda Hamilton's character, Sarah Connor, is trying to escape the mental hospital with the Terminator and her son. However, the T-1000 is in pursuit and follows them down an elevator shaft. The scene involved Hamilton firing off a handgun into the roof of the elevator with extra-loud blanks - which she did without wearing ear-plugs to minimise the damage. When you watch the scene again, you can see Hamilton grimacing just after she's been cut and the earplugs dropped out as she was bending down.
1. LORD OF THE RINGS: THE TWO TOWERS - Viggo Mortensen broke two of his toes
Viggo Mortensen is one of these actors who really is able to embody his characters and give them real life and personality. However, on Lord of the Rings: The Two Towers, Mortensen didn't need to access his acting chops to give off a roar of anguish. In the scene where Mortensen finds a pile of dead Urak-hai and thinks the Hobbits are dead, he kicks a helmet in anger and lets out a roar. As it turned out, Mortensen actually broke two of his toes, shouted, and then finished the scene. Jackson thought the shout was just an ad-lib, but it actually turned out to be real.