In PHX many years ago at a CO gate, a confused looking Captain and First Officer came running up to the gate then stood at the window looking at the plane. Then they walked back and looked at the gate monitor, then the paper work, and then the plane. Of course we were all looking at this point wondering what they are doing. Finally the Captian said that is not a 737. The First officer said, I can't believe they swapped aircraft out on us again, we are not rated to fly that aircraft! Several of us heard the exchange and looked at them with bewilderment. They stood there looking confused at the plane and paper work and talking.