When Martin (John Mahoney) starts his card trick, the Jack of Diamonds is seen at the bottom of the deck. After the trick, when Niles (David Hyde Pierce) draws the Ace of Spades out of the deck, the 4 of Diamonds is seen on the bottom although Martin never shuffles the cards.
When Daphne mentions she rotates Martin's underwear by throwing out a pair each month, then putting in two new ones, she says: "did it ever occur to you you never had to buy underwear in ten years?" Even if she started doing this not exactly 10 years ago, Martin's underwear collection would have increased by over 100 pairs with this system, which doesn't seem like something nobody would have noticed until now.