Despite having a menu full of solid staples, Shake Shack is no stranger to experimenting. While celebrity chef David Chang caused a record-breaking Shake Shack line with his Shrimp Stack burger, and the restaurant once offered dragon-themed foods during the run of the eighth season of "Game of Thrones," there's one ingredient that's simply too much for the burger chain's CEO.

"I think you can use a burger as a canvas for anything sensible. But I do draw the line at peanut butter," said the Shake Shack founder and current chairman Danny Meyer after learning that his chain was selling a burger topped with bacon and peanut butter. Whether the idea of a peanut butter burger makes your stomach rumble or inspires a sense of morbid curiosity, it's too late now to try Shake Shack's take on this creative combination.

For years after the sandwich was discontinued, clever customers would order a side of peanut butter sauce and drizzle it over a ShackBurger. However, this hack was made impossible in 2018, when Shake Shack removed peanut butter from the U.S. menu due to allergy concerns.