Ricky's Old Girlfriend - S3-E11
Continuity mistake: In Lucy's dream, when Carlota Romero comes to their apartment, Little Ricky is sitting in a highchair with a hat on his head. Although later in the scene, you see his hat fall off, the first close-up shot of Little Ricky shows him without the hat. Then immediately, when you see a wide-shot of the scene, his hat is back on his head again.