I found the problem: if I already have an item in my cart, then go back to the product page and try to add it, I get the error “Maximum allowed quantity for PRODUCT_NAME is 1. Your current item quantity is 2.” It’s confusing, though: I’ve set my quantity to 1 (on the product page) … the error should be clearer, perhaps something like “Maximum allowed quantity for PRODUCT_NAME is 1. You already have 1 PRODUCT_NAME in your cart.”