I recall the Ewoks' appearance being spoiled on tv. (Can't remember if it was local or network.) This was after months of major secrecy. Kenner had blacked out their pictures in the toy catalogs, even the cardbacks, and IIRC, photography was not allowed in their toy fair booth that year.

Some ROTJ puzzles, (not by Kenner) hit store shelves ahead of the film, and a news report on the pre release mania flashed a picture of Artoo and Wicket that was on the box. Damn you mainstream media! ;)

On the flip side of the coin, none of the other aliens really were kept secret, except for Jabba. (Admiral Ackbar was the mail away "preview" figure much like Boba Fett was before ESB.) Boushh was listed as a bounty hunter, but the true identity was preserved until after the movie was out.