Oh ... you must mean this:
IMDb - THX 1138 (1971) - Trivia
http://www.imdb.com/title/tt0066434/trivia
"A scene in which THX falls into a garbage compactor and fends off a mutated rodent was cut because the monster did not look realistic. This situation was later reused in Star Wars."
Of course, now that George can make "realistic" monsters, expect to see yet another release -- THX 1138: The George Lucas Guaranteed Original Director's Cut. :P