They're assholes about allergies/dietary restrictions
Take a deep breath, because this is the big one!
Servers and restaurant employees will argue incessantly with each other over the subject of allergies. A lot of servers try to shift blame back to customers who claim they’re allergic to something when they just don’t like it, which, OK, that’s a lame play, but it’s also not an excuse for a server to just assume everyone is doing that: someone I know recently claimed that 99% of customers who claim allergies are just being picky, which is some pretty irresponsible math.
As soon as the server makes that assumption, the customer ceases to be the asshole. At a restaurant in Frederick, Maryland, I worked with one cook named Chris who liked to claim that he had learned in “culinary school” that human beings could “only be allergic to like five or six things,” and anyone who claimed an allergy to something outside those boundaries was lying. The moral here is don’t be that kind of asshole, restaurant employees, because someday one of your coworkers might out you as a complete douchetart on a major national food website using your actual first name! Or you might seriously make somebody sick or dead. Either way, don’t be Chris. Chris sucks.
When I submitted this subject to my server friends, one of them used an excuse I’ve heard before but never even slightly bought: restaurants, when informed of an allergy, clean everything and use separate utensils and basically put on Cirque du Soleil in the kitchen in service of the customer’s issues. Look, I’m not going to say this NEVER happens. There probably do exist places that actually do this, somewhere, and if a kitchen is forced to decontaminate because a customer is full of shit, that customer deserves ire. But I worked in a lot of different restaurants, and I never saw ANY of them get particularly antsy even about cross-contamination, let alone the production described herein. It’s just not a thing that happens frequently (even assuming it happens at all). But if it does, it's probably for a very good reason: safety.
Bottom line: just don’t be a dick about people who claim to have allergy issues. Always assume they’re legit. If they aren’t and you believe it, sure, the customer is an asshole, but that’s all that happens. If they ARE and you don’t believe it, you could freaking kill someone, which definitely merits some anger on the part of the customer. Getting to hold onto your feeling of moral and intellectual superiority isn’t worth that possibility.