[BobaFloutist]

They don't need to make all the food with that level of accuracy, only meals called out as necessarily allergy free.

Restaurants have specific procedures for allergy meals. It's not that hard to do an extra good job when someone discloses an allergy.

[crazygringo]

Yes, restaurants have specific procedures for allergy meals. But restaurants have specific procedures for everything, and procedures fail constantly.

Saying you have an allergy doesn't mean the server won't simply forget to pass it on. Or won't misremember the ingredients in a dish. Or that a runner won't actually grab a dish for the wrong table.

There are no magic order-to-serving procedures that are guaranteed to work 99.9999% of the time that are also easy to implement in a cost-effective way. It actually is really hard.

So what happens is everybody tries their best but there are just no guarantees, and mistakes get made all the time.