Accidental surprise setting off of small stunt improvised explosive device - causes camel to balk and knock actor flat onto his front - hits his head on a large canister of milk, knocks it totally over sort of backwards onto him, he ends laying in large puddle of milk, then ten hungry feral neighborhood cats appear screaming and fighting over milk - The Hurt Locker
4 minutes of general hilarity / corpsing when Colin Firth says 'knob' in A Single Man
Sandra Bullock's stunt double trips over and misses an attempt to fall down a well, instead trips over own feet, hits head on grass, passes out with ass in air. All About Steve.
Blue skin CGI pigment turns to green, then red, then black allofasudden, but in 3D - Avatar