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When the bun disintegrates. Usually, it's the too thin bottom half when way too much moisture is placed on it. You're left holding a mess in your hands. This is a failed combination. Don't use the cheapest buns and don't add a ton of watery crap.

At sit-down, non-fast food joints, the trend for over four decades now has been to overdo it with combinations that are more upscale. The ingredients are mostly fine, except for they stack all that shit too high. It's one more way the thing just immediately fails.