And the Fraggle Rock characters in Jabba's Palace, singing that ineffably stupid "Jedi Rocks" song. I don't know anyone who thinks that was a good idea.
On a slightly lower level of stupidity, but still quite awful, are the beak on the Sarlaac and Vader's unnecessary shuttle trip. The former looks like claymation and totally undermines the "ominous" feel of the scene, settling instead for cheap laughs and "wow" factor. The latter disrupts the entire pace of the chase scene and adds totally unnecessary exposition.