In Back to the Future Part III (1990), this movie, Ma and Pa Kettle at Waikiki (1953) and Abbott and Costello Meet the Mummy (1955) are playing at the Pohatchee Drive-In Theater in Hill Valley on November 16, 1955.
With Donald O'Connor's DUKW (duck) having stalled for want of gasoline, Francis comes along and asks him, "Wanna sell a duck?" This clever play on words was a reference to a catchphrase of the late comedian Joe Penner, "Wanna buy a duck?"
While speaking to Lieutenant Peter Stirling/Boatswain Mate Slicker Donovan (Donald O'Connor), Francis turns his head for only a moment to a wall, at which point his voice echoes. It appears that as Chill Wills' dubbed in the mule's voice, a rather painstaking detail was considered.