When Giles lands at the airport in New York and drives thru the city all the trees are in full leaf as they would be in summer yet when he takes a cab to the motel on Long Island here are no leaves on the trees and it is obvious it is a different season.
When the mailman delivers mail to Ronnie Bostock's mailbox,he raises the mailbox flag, presumably to signal to the resident that mail has been delivered. (Ronnie's girlfriend, seeing the mailbox flag has been raised, seems to interpret the signal accordingly.) Although it may be the convention for mail delivery wherever the director/writer is from, it is not the case on Long Island, where it is the custom for the resident to raise the mailbox flag to alert the mailman that mail is in the mailbox waiting to be picked up. Once the mail has been picked up, the mailman lowers the flag - the opposite of what occurred in the film.