As Lanney signs the bill in the hotel room when Maureen brings him his food, there is a ZIP code visible in the hotel's address. This part of the film is set in 1957, but ZIP codes were not used by the US postal service until 1963.
When Lanny and Karen are having dinner and they leave the restaurant Karen has hoop earrings in (both ears) but when she goes back and Lanny kisses her she has no earring in her left ear, though the one in her right ear is still visible and she had them both on throughout the rest of the scene.
The dates given in the movie are inconsistent. The telethon takes place Veterans Day weekend, November, 1957. It is stated that telethon finished on noon that Sunday, which, according to the calendar, would be November 10th. However the date on the room service bill signed the previous afternoon for Maureen is also dated November 10th. And when the duo are asked to open the New Jersey night club on November 15th, they respond they can't because that is the next day after the telethon ... and that date is inconsistent with both the calendar and the room service bill.