5. Ordering nothing but water and staying all day
You've seen this dude. He'll park at a six-top table, grab a glass of water, then unload the contents of a backpack on all six spaces at the table, plug his computer into an outlet three tables away (creating a tripping hazard), then sit there all day drinking water like a camel. When he leaves, the table's covered in empty glasses, torn paper, and crumbs from the snacks he packed from home. Somebody has to clean up after him. That person's not very happy.