While I do agree with the OP, I understand that people do make mistakes. I also agree with Norcal, but in the same instant, my job in the military is like the fast food industry where when someone gets a wrong part, they flip out on me, like it's my fault that the Depot sent them a broken widget, or that the battery they received from a contractor wasn't charged like it was supposed to be, or even better, when the maintenance org. gives me the wrong stock number and then they bitch at me for ordering the wrong part when all I did was order exactly what they told me to, such is the plight of being a middle man.
A lot of times, the cooks aren't the ones taking the flak in the fast food industry, but the person you look at and give the order to. Granted, if that person doesn't press the right button, then it's going to be wrong regardless. That can't happen in my career field, because a wrong button press will order a completely different part.
As far as the OP goes though, all I can say is that I despise mayo, and everywhere, including Wendy's, I ensure to tell them no mayo, because the Wendy's I go to, usually soaks everything in mayo before wrapping it. Well about half the time I get a chicken sandwich soaked in mayo. So much so, that I've just started ordering everything plain, just so I don't have to put up with the mayo, I split the veggies on my wife's sandwich just to get what I'm missing.
However, I once ordered a double cheese burger at Burger King, in the dining area mind you, I ordered it plain because I wasn't in the mood for someone to fuck up, and I got to hear the cooks in the back start asking the cashier, if a cheese burger plain has cheese on it. The cashier looked at me and asked me. The people working were around my age, and the only customer's they were serving was I and my roommate. There was no one in the drive thru and no one else in the dining area.
Also, not that I have any food allergies but that is something to take into account. Someone may be ordering no onions because they are allergic to them, you just never really know when someone is taking off stuff that normally comes with a meal.
My 2 cents.