I'm not sure how you define 'the worst mistake ever' where food service is concerned. Here's a few things I can remember:
1. The restaurant, in Brighton, where the custard on my pudding came with a sliced green bean in it ;
2. The (obviously new) silver service waitress, in a posh restaurant, who took great care to serve me from the correct side. She then struggled valiantly to lift the massive Dover sole from her platter, using a fish slice, without it breaking up. She carefully slid it onto my 'plate'. It was only at this point that she realised that the design on the table mats was the same as the design on the plate (which she'd forgotten to bring)! i.e. one beautifully cooked Dover sole was now sitting on the table mat ;-)
3. One Sunday lunchtime, I was working behind the bar in a pub, when a couple came in and asked if they could have lunch. I said that I thought our restaurant was fully booked but I'd check with the kitchen. I returned to advise them that we could only serve them with sandwiches. They said it would be fine, and ordered two cheese sandwiches.
About half an hour later, they asked when they'd get them. The boss, in the kitchen, said that they wouldn't be long.
Another half an hour went by before the same question was met with the same response.
Half an hour after that, the boss said "We've got no butter for those sandwiches. Can you drive to the nearest shop and get some, please?". I had to get several customers to move their cars, so I could get mine out of the car park but I eventually returned with the butter.
Another long wait occurred, before the landlord said, "We've got no bread. Would you mind . . .?" After another struggle to get out of the car park, I fetched the bread.
Ages later, the landlord said "You'll have to go to the shop again. We've got no cheese!". The customers ordered their