A fast-food restaurant's success is based on a consistent menu of perennial favorites, right? Not always. Ever seeking a way to stay current, fast-food establishments often introduce items that try to speak to the trends of the times. Apples in Happy Meals? We're all for it. But some product introductions just don't cut the mustard. We're taking a look at some of fast food's most famous flops — some of which never made it to the U.S., while others have attempted several comebacks — with little success.Is fast food one of your guilty pleasures? Top chefs reveal the fast-food restaurants they're not ashamed to love . When you're ready to take a break from fast food, check out this great collection of healthy, fast-food alternatives