Erin's lipstick when she runs into Alan on the train. When she falls into his arms she has none on, then when she finally steps away from him, she has on lipstick.
The level of wine in Erin's glass, when she is dining with the Brazilian.
In the first scene, wherein Erin is being dumped by Sean, the cat in her arms keeps switching positions between shots.
When Julie meets Alan in the restaurant, when she sits down, she immediately uses a napkin to dispose of her gum. A few moments later, she gets rid of her gum a second time.
The MBTA (subway) map at the beginning of the film is wrong. It also doesn't correspond to the subway route used in the movie. The conductor says the next stop inbound is Aquarium from Airport. The map says the next stop should be Maverick (which it is, in real life).
The subway map of the MBTA Blue Line shown inside the subway car at the beginning of the movie shows five stops between the Airport station and Wonderland station (as there are in real life). At the end of the movie, the train operator calls out "Next stop, Wonderland." This skips all stops between Airport and Wonderland. There are no such express trains on the MBTA Blue Line.