At last minute, Jan (a female movie star) makes a surprise appearance in a role in an all-male West Point revue, in a part everyone assumed was to be played by a man. But a few minutes later there's a photo of her in closeup in the program featuring stars of the production.
Hal's knee is injured backstage during the last act of revue, forcing Bix to take his place in the big final number. Yet during the finale, Hal has recovered enough to perform the next-to-last reprise number, which is now climaxed by Bix's final number, which was originally to have been performed by Hal. Had Hal not been injured, how could he possibly have performed both numbers in the finale?