When Erin is entering the airport to leave New York, she is pulling one suitcase. Once her sister picks her up in California and they are exiting the sister's vehicle, Erin is pulling one suitcase and her sister is also pulling a suitcase.
When Garret and Box are discussing why Box grew his mustache, there are several references to the 1970s, including "Burt and his Camaro", which is an obvious allusion to Burt Reynolds in Smokey and the Bandit. However, the car Burt drove in all of the Smokey and the Bandit movies was a Pontiac Trans Am, not a Chevrolet Camaro.
In the "morning after" scene in the diner, while Erin is explaining why she doesn't want to get involved in a relationship, Garrett drinks his entire glass of water. In the next full shot of the table, his glass is full again.
When Garrett drives Erin to the airport following their argument, Garrett is driving toward the Terminal 1 departure level at JFK International Airport. Terminal 1 is an international-only terminal and as such would not serve any flights flying to San Francisco.
In the scene where Garret leaves California after his visit, he states that he "got the middle seat" after getting his boarding pass from Southwest Airlines. Southwest boarding passes do not assign seats, but have boarding groups instead, so he could not know what seat he would be taking.
In one scene at the airport, it appears as though Erin walked Garrett to his gate and they sit there to wait, however, it would not be possible for someone without a ticket to make it past security and to the gate.