The water taxi that Charles, Sebastian and Julia take to Lord Marchmain's palazzo in Venice was built post-1960s. All Venetian water taxis before this were of solid wood construction, but the one used for filming is clearly of "cold-molded" construction, as one can readily see by the diagonal strips of veneer on the hull; something unknown before the advent of resin laid-up veneer hulls in the 1970s.
The Venetian drummers have instruments fitted with Mylar drumheads (Remo Pinstripes) that were not manufactured before WWII. Calfskin heads or ones that resembled calfskin would have been more appropriate for the time.
In Lord Marchmain's deathbed scene, Fr. Mackay imparts absolution while Charles Ryder and members of the family are in attendance. Absolution is never imparted in public in this way. The others would have been asked to step out. Moreover, the Latin form of the absolution given, although it is the correct traditional one, is badly mispronounced and contains several errors in the details of the Latin text.