Angela and Oliver bring a wrapped Christmas tree to Angela's home. They don't unwrap it or decorate it. But the next morning the tree is fully decorated with ornaments and lights.
Close to the end of the movie, Angela comes to the hotel and meets the Fletchers. It was afternoon. They decide to go right away on Vespas to St. Peter's Square. In the next scene they are driving Vespas but suddenly it's evening and dark.
In the 3rd Act, Oliver left his hotel with a bag and carry-on luggage, and went to St. Peter's Square. However, at St. Peter's Square he was without them.
Caffè sospeso is not a Roman Christmas tradition but an anonymous act of charity that started in the working-class cafes of Naples.
The Italian plural of espresso is espressi not espressos.
Oliver tells Estelle he is taking the "red-eye" from Italy to New York. There are no red-eye flights from Europe to the U.S. Those are only West-->East.
Rome has green deciduous trees with green leaves in December, but not every single one of them, and also some trees have to have yellow leaves in December if they haven't completely lost their leaves. The movie was clearly not shot in December but in spring or early summer as outside of Forlinghetti's company you can hear loud songbirds singing from nearby trees, and they don't sing in autumn and especially in winter.
Oliver is shown carrying his heavy travel bag around Rome, but it clearly has wheels and an extension handle.