johnny and the bomb - What's the worst thing anyone could do in a fast-food restaurant?

In Johnny and the Bomb (chapter 8) Terry Pratchett writes this:

Sir John sat down heavily in a seat, motioned them to sit down as well, and then did the second-worst thing anyone could do in a fast-food restaurant.

He snapped his fingers at a waitress.

All the staff were watching them anxiously.

‘Young lady,’ said Sir John, wheezing slightly, ‘these people will have whatever they want. I will have a glass of water. Thank you.’

‘Yes, Sir John,’ said the waitress, and hurried away.

‘You're not s'posed to do that,’ said Bigmac hoarsely. ‘You're s'posed to queue up.’

‘No, you're supposed to queue up,’ said Sir John. ‘I don't have to.’

We learn that Sir John gets away with this because he's the billionaire owner of the entire burger franchise.

But I feel like I'm missing a joke here. Perhaps I'm bad at British humour. What's the worst thing anyone could do in a fast-food restaurant, the single thing that's worse than snapping your fingers?

I looked in the Annotated Pratchett File, but this joke isn't explained.

Answer

Maybe I've read too much Pratchett and Adams in my life, but to me seems simply one of their typical jokes.

More or less every book ever written has some instance of "the best thing", "the worst thing", etc. (just think that Apple entire advertising is based on that trope), here the joke is that he did "the second worst". It is a joke on the constant abuse of the same trope over and over again in literature, and it is something really typical for Pratchett.