I would just like to take this opportunity to say that to the best of my knowledge, this has never happened anywhere in food service that I've worked. The only time I've ever seen anything remotely like it was when I worked at a pizza shop and this woman INSISTED I'd made her wings wrong. I hadn't. We had five wing sauces: barbeque mild, barbeque hot, garlic butter, buffalo, and atomic, which was all the parts of barbeque hot except the barbeque sauce and with added pureed jalapenos, which in Yankee country counts as "super spicy". This woman asked for atomic and swore I gave her mild. You'll notice that there's pretty much zero percent chance of mixing up barbeque mild--served straight out of the BBQ container--with atomic--which had several ingredients that had to be mixed by hand--without noticing that something is very wrong by virtue of color, consistency, and barbeque mild not making your eyes water if you smell it.
I very politely apologized, took her order into the kitchen, put everything back into the not-yet-emptied shaketub I'd used for her order, thumped it around a bit, and took it back out. She bit into one, went "mmmmm, now THAT'S what it's supposed to taste like" and took her order.
I HADN'T CHANGED A THING. No "special ingredients" (GROSS), no new sauce, nothing. Some people truly do want to just complain, but you don't rise to the bait. Period.