I'm interested in traveling from Washington National (DCA) to LaGuardia (LGA) on the same day; and simply turning right around and returning on (I think) the same aircraft. There is a 50-minute time from the scheduled arrival in LGA and the scheduled departure of the craft. Will there be a problem (1) if the plane from DCA is late and I don't "check in" in the requisite time window? (I don't want to be stranded up in New York!) or (2) that I don't check in at the check-in counter?