Marriage ceremonies are steeped in history, and with so many cultures, religions, regions, and couples adding their own customs into the mix, it should come as no surprise that wedding-related traditions and superstitions have emerged over the years. According to folklore, a knife signifies a broken relationship and is bad luck to give as a wedding present. If knives are on your registry, just give the gift giver a penny. That way, it's considered a purchase.

Source: Martha Stewart