Off-menu items are like the hidden door in the bookcase -- you've got to be able to tell cashiers exactly which button to push on the register in order to reveal the hidden wonders at your favorite fast food restaurant. Mental Floss published a list of 10 off-menu items at popular fast food chains -- but as with so much of the Internet, the best information is in the comments and suggestions that follow the story. A double grilled quesadilla at Taco Bell is a good idea. The chili cheese burr