On the Death Star, Luke was sucked under the surface of the sewage water in the trash compactor by a dianoga. Han and Leia tried frantically to pull him out, but couldn't find him. The water only came up to about their knees, though, so why did they have so much trouble finding Luke?
Note: the idea for this question came from http://www.moviemistakes.com/film1226/questions/pageall.