I once had a dream really similar to this. It was for a multi-year long crucial to the fate of humanity expedition of some kind though and I was supposed to have been training for it with my shipmates for years. Somehow I forgot this until the day of launch itself and I tried to sort of wing it but the crew and especially the captain gave me some really filthy looks and were really pissed that I was jeopardizing the mission with my amateurism and made a mockery of all the work they put in training for the past few years.