According to Wookieepedia, in the original 1977 Star Wars film (now known as A New Hope), after the bounty hunter Greedo says "I've been looking forward to this moment for a long time" (talking about killing Han Solo in Mos Eisley cantina), Han shoots Greedo under the table while Greedo was pointing a blaster at him. You can read a lot more discussion of the controversy at "Han shot first" on Wookieepedia.