Star Wars Episode 6: Return of the Jedi - See Through Falcon
In the old Copies of ROJ there is a scene near the end of the movie where the Millenium Falcon goes through a large Formation of Tie Fighters, if you step through this scene frame by frame you will see the white silhouette of a tie fight. It has been removed in the special editions.
Actually this was explained in one the 'Making of Star Wars' videos available. Apparently, it had something to do with the way they used matting to transpose the images of the Tie Fighters onto the screen. They left in in because it was so brief.
It also happens during the Landspeeder/AT AT fight. When Luke's speeder is crashing into the snow by the AT AT leg you can see the AT AT's leg through the 'transparent' left hand window strut on the speeder.
It's not really a slip-up. At the time that the film was made this was the only way that they could do the affects (no cgi's) plus Lucas explains all about it in the Making of video's on the special edition.