During the "Madonna" scene at about 1:04, the amount of tape on the reels of the recorder changes between the wide shots and the close-ups.
When Marcello and Maddalena come out of the prostitute's apartment their car is already running (exhaust is visible at the back) before they get in.
The aircraft which brings Sylvia to Rome is an Alitalia Vickers Viscount as it comes in to land, but is both a Douglas DC-7C and DC-6B when it is on the ground.
When Marcello is typewriting in a restaurant on the beach and talking to the young blonde girl, the bar of the typewriter is centered on the machine. In the next take, it is displaced to the left of the typewriter.
When Marcello and Maddalena arrive at the prostitute's apartment, a long electric cable (light?) can be seen attached to the right rear of the car, moving along until the car stops.
In the castle, Marcello lights a match to view some ancestral paintings. A spotlight is used to enhance the light of this match, but does not follow the actor's movements very closely and gives the illusion away.
At the top of St. Peter's Dome, the wind blows Sylvia's hat off. The wire on her hat used to achieve this effect is visible trailing off to the right.
In the Via Veneto scene when Marcello meets his father, the windshield of Marcello's car is missing; his hand visibly holds onto the windshield frame as he exits his car.
When the two children see the Madonna, the time of day is stated as 7:00. However, the very short shadows of the characters reveal that it is midday.
The water in the Trevi Fountain continues flowing even after the sound of it has faded away, and only stops when the camera changes view.
After leaving the club where Marcello meets Fanny, the occupants of Marcello's car are singing Stormy Weather, but their mouths are singing something completely different.