READ THE MENUS! I one time had a customer order a Turkey Club WRAP, but he only said Turkey club. This is in the 'WRAP' section of the menu and we only have it in a wrap, not as a sandwich. I bring the food out and the customer got mad and said "this isn't what I ordered", to which I was very confused and looked through my pad to see what I may have did wrong.
I saw I wrote "TC" (Turkey Club), so I knew I didn't do it wrong. I went back out to customer and advised him that what he ordered was a Turkey Club WRAP and we don't have it as a sandwich, but we can put it on bread, if you wanted us to do so. Customer refused and said he doesn't want it or anything else. Got an 8 cent tip on that bill.
TL;DL - Customer didn't properly read menu and ordered something he thought was something different, and I got a shit tip because of it.
Or when they come in and just make up their own menu item or combo of smaller items and expect you to be able to just make it work