Remember to have your I-20 on your person and not packed away in checked luggage since you go through inspection before claiming bags. That said, if you forget your I-20, you should ask the immigration official for an I-515 upon entry, which will allow your admission in F-1 status for 30 days. With an F-1 visa you are normally admitted to the U.S. For 'duration of status' (D/S,) but if you are missing the I-20 it is most likely you will be admitted for 30 days with an I-515. Within that 30 days you will need to come to the ISSO for another I-20 and will need to send the new I-20 and the I-94 admission proof (departure card completed on the plane or print out from new automated system) to a government office to change your stay to “duration of status.” In a pinch you can log into SSOL to print out your registration record to demonstrate full-time enrollment.