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.
One day you don't show up for training, someone asks where you are and somebody says "I think they are training at another facility or something". Later that week a head honcho asks where you are and are told you are training at another facility. Not wanting to lose face in front of the troops they just nod and accept it.
They ask an aid to find out what facility and to get the training results to the science team so they can integrate it. The aid goes crazy as either no one knows or they just say any information about personnel is classified. He puts in a lot of effort to get the right clearances and get the data, but is unable to do so. Every time his boss asks he doesn't know so he gets chewed out.
At some point someone figures out what happened, but its too late, if it comes out heads will roll. So they think whatever, come launch day we'll just put in the backup and it'll be fine. But Murphy strikes and the whole backup team gets measles, at least that's the official version (in realty they all got chlamydia). So there's no choice but to put you in. Through the grapevine the crew finds out and is ultra pissed, but no wanting to let humanity down they roll with it.