It drives me fucking insane when I see my tables stopping other people to order from them, or walking up to the bar to order their drinks. Now, if I'm totally slammed, that's one thing. If I've just been at the table, there's no reason to flag someone else down to order a salad. Newsflash: You're still going to pay for it. I'll see you eat it, and I'll make sure it's on your ticket.
Going to the bar is another dumbass thing. I had a table last night that had literally just sat down -- in fact, three out of four were seated, the fourth didn't even sit down before going to the bar to order his beer. I was approaching the table as he walked by me to order from the bartender. I felt like grabbing him by his poofy, scraggly mustache and dragging his ass back to the table. Because the bartenders don't need any more sales. They get away with ringing in groups of bar guests under one guest number, and they by default have a lot more drinks; so they're already at the top of the damn upselling game. I need that point for a beer under my name so I don't get fired for something retarded!
And that aside, since I know most customers wouldn't have a clue -- it's just plain fucking rude.