Clothing, footwear and accessories. This is the category with the most codes in circulation, and also the one where conditions matter most: first-order codes, student codes and member-only codes all sit side by side, and only one of them will apply to your cart.
Two exclusions repeat across nearly every fashion retailer. Outlet and clearance sections sit outside the scope of percentage codes, and newly released lines are usually excluded for their first 30 days. If a code silently fails, one of those two is almost always the reason.
Member pricing is the other trap. Several retailers run a member price alongside promo codes and make the two mutually exclusive at checkout, so the cheaper option is not always the one with a code attached. Compare the total both ways before you pay.