Came up with some of these on my own, heard some, and my sister's boyfriend helps when we have Star Wars' Movie Night (second saturday every month!)... and sorry about the length...

***When Leia is shot by a stormtrooper at the shield generator, Han reaches down and grabs her shoulder. He then quickly reaches down and grabs her breast.
***As they take Chewie away in Jabba's Palace, watch Lando. Just before he tilts his mask down so we can see who he is, he hits his head on the doorway.
***When Oola, the dancing girl, is trying to get away from Jabba, she briefly falls out of the top of her costume. It isn't as visible in the widescreen version, but quite visible in the regular version.
***In one of the scenes where Vader and the Emperor are trying to turn Luke to the Dark Side, the Emperor is trying to get Luke to "strike him down" with his lightsaber. Luke finally gets annoyed enough to give it a try, but as he swings down to slice the Emperor in half, Vader suddenly whips out his lightsaber and blocks the blow; then the camera cuts to a nifty closeup of the crossed lightsabers and the Emperor laughing. However, according to that shot, the Emperor should be dead. Look carefully at the position of the sabres: Vader's is on top of Luke's - a position which should have driven Luke's lightsaber straight into the Emperor's body.
***Just before the rebels launch for the Death Star Battle, the Admiral says "Prepare for hyperspace...at my mark!" then it switches to Lando in the Falcon and he says "Roger. Stand by" then IMMEDIATELY pulls the hyperdrive lever and FOOOM!! Off they go. Wasn't he supposed to wait for the command first?
***In the scene where Han Solo is running from the explosion of the shield generator on the Ewok planet, you can see his reflection in the plexiglass that is used to protect him for the explosion.
***How can rocks thrown by the Ewoks injure the stormtroopers?
***The biker-scouts ride through Endor on speeder bikes. These things go 500 kph+, in those trees they would have crashed before they got five feet.