Just before the "Isn't It Romantic?" number begins in the tailor shop, Maurice reacts with pleasure as his customer Emile steps out of the dressing room, supposedly wearing his new suit. But in the mirror's reflection we can see that actor Roach is still wearing his long-johns from earlier in the scene. In the next shot, he is suddenly wearing the suit.
Jeanette's hairstyle changes slightly when she goes out into the garden, during the party.
The lighting on Jeanette during the balcony scene.
Camera shadow on a curtain as it pulls out of a window, as Jeanette sleeps (a little after the "I fell flat on my flute" sequence).