Star Wars Bloopers
Episodes IV, V, VI
Miscellaneous mishaps and oddities from the first three films. My favorite part: around 3:15, an AT-AT catches fire, revealing its true scale.
R2-D2 has serious stability problems. Bonus: His lines, Yoda messes up.
Anakin face-planting, falling down during a lightsaber battle, and spitting out overly hot soup.
There was "Nooooooooooo!" official blooper reel for Episode III, so some YouTube person cut together behind-the-scenes footage of wacky stuff. My favorite? C-3PO falling down unintentionally, and Samuel L. Jackson falling down intentionally.
If Star Trek is more your thing, check out my collection of Star Trek bloopers.