The harmonica that Cinzia wins for Robert is labeled "ECHO". An echo harmonica has 2 reeds for each note, each reed slightly detuned sharp and flat to provide a tremolo effect. The sound when Robert is playing this instrument is of a normal harmonica.
When Tom comes outside after hurriedly dressing to discover the houseboat has broken loose, his shirt is unbuttoned to his waist. In the following shot a split-second later his shirt is completely buttoned all the way up to his collar.
During Cinzia's first visit to Tom's apartment, at one point he is adjusting the sofa bed; as the two converse the camera moves closer to them and the shadow of the moving camera can be seen on Cinzia's dress.
Tom's apartment is in Washington, D.C., but when he's shown driving his kids to the concert at the Watergate, he approaches the area from the Virginia side of the Memorial Bridge, the opposite direction from which he should logically have been coming.
When driving to the house, singing in the car, they are pulling a small trailer with their belongings. However the shots from the front angle (with the rear screen process) show no trailer behind them.