I Love Lucy S03 E09 Too Many Crooks

"Vitameatavegamin - Spoon Your Way To Health!"

Lucy sneaks into the Mertzes' apartment to borrow one of Fred's suits so that she can order him a custom-made tweed suit for his birthday. This leads Ethel to think that Lucy is the notorious neighborhood burglar, "Madame X", and she goes spying on her friend to prove it.