The entire clip is worth watching all the way through, though some highlights include C-3PO tumbling down a Tatooine sand dune, a series of Storm Troopers getting stuck in a door then slipping on a metal grate, and Harrison Ford flubbing a line and then pretending to eat his headset microphone in frustration.
Perhaps the funniest moment comes when Sir Alec Guinness is just about to deliver a line aboard the Millennium Falcon only to have Chewbacca's hairy arm brush in front of his face. The exasperated actor raises his eyebrows and widens his eyes in surprise and annoyance, then attempts to say the line, only to be greeted with the film crew's laughter, causing Guinness to break, too.
We can imagine it was probably pretty hard for many of the actors to deliver such strange lines of dialogue with a straight face -- see: Mark Hamill's confused repetition of the word "supernova" toward the end of the clip -- so there's probably a treasure trove of similar blooper reels hanging around somewhere. "Star Wars" fans can tide themselves over with this clip for the time being.