My friend gets the worst customers come in to his chip shop.
A woman once started taking an order off the counter, and when asked if it was her actually her food, said she hadn't even ordered yet. WTF?! then it's not your food is it?
Another guy who responded to "number 13, one portion of chips", only to complain that it wasn't what he ordered, and that he was number 20.
At my job in a restaurant, people forget what they've ordered all the time. Once I had three bowls of soup for a table of four, and every single one of them claimed that they hadn't ordered any soup. After a bit more discussion, they remembered that actually, 3/4 of them has ordered soup, and just forgotten in the 5 minutes it took for me to bring it over.
We also get people complaining/asking me to bring them stuff that's already on the table. Sometimes it's small things like mustard, sometimes it's something glaringly obvious liek a huge dish of vegetables that they just don't see because they're so hell bent on looking for something to complain about.
I've had people click fingers at me, but worse than that is when they say "well done", as I put things dowen on the table. Well done? that's more than a little bit patronising.