It's not groundbreaking to say that working with the general public can often be a challenging experience. For every sweet person who makes the customer service experience almost worthwhile, there are several nightmares who think nothing of treating those offering them a service with the utmost disrespect. Servers in particular often have to deal with demanding customers—hangriness really brings out the worst in some individuals.
One annoying but common occurrence is customers who refuse to let a server finish greeting them before throwing out their drink order. This was recently a topic of discussion on Reddit, where wait staff shared all the different methods they had of shaming those who chose to be impolite in this way. It's fair to say that they listed some effective ways of making those entitled people think twice about being so snappy. It doesn't take a lot of time and effort to show basic manners.