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Most people go through life trying not to offend others. There are surely some who have ill intent, like those who play slot machine games on their cell phone while on public transportation with the volume turned up, but the majority of us try not to be rude. It's generally pretty easy to not be an abhorrent person, but there are a few things customers might do at a restaurant that could make the server perceive them as just a teensy bit disrespectful.
Don't assume your server does something other than wait tables for a living.
Maybe you're trying to initiate small talk to fill that awkward five seconds of silence as your server refills your water glass, but be mindful of what you talk about. There are better topics of conversation than asking if they are in school or what else they do for a living because those questions imply that waiting tables somehow isn't enough. You don't want to unintentionally offend someone whose sole purpose is to serve you. "How is your day going?" is good. "Do you have a real job?" is bad. Like, really bad.
No one knows the menu better than your server.
Plenty of restaurants require a battery of menu tests before the server is allowed to swaddle themselves in that most precious of aprons. Those servers also know which menu items get ordered the most by customers and which ones languish on the prep shelf. If you want an opinion on what to order, your server is the one to ask. When they tell you the salmon with the hint of smokiness from the grill is delicious and is by far the most popular entree on the menu, it's hurtful when you wrinkle your nose and order a hamburger instead. At least try to pretend you're considering their suggestion. Imagine your closest friend asking you which of their outfits you like best and as soon as you proclaim your choice they immediately choose the other one. It stings a little bit, right?
A table isn't your trash bin.
There's nothing wrong with leaving a few things at your table when you're finished eating, but not all discarded items are created equal. A stray gum wrapper? Sure. A random grocery store receipt? No problem. The entire contents of whatever was at the bottom of your purse along with a used Kleenex and a couple of old N95 masks? Absolutely not. Your server is going to clear the table after you leave and likely drop a good deal of the things into a garbage can, but leaving enough of your own personal trash to require extra trips back to the table is rude. If there was an exchange of presents during the course of your meal and you want to leave the wrapping paper, tissue paper, ribbons, and cardboard boxes, recognize that you're leaving it. The restaurant doesn't expect you to carry it out, but it is nice when you acknowledge that you're asking your server to take care of it for you.
Personal space is personal.
Some people simply aren't the touchy-feely type and the thought of a stranger even politely placing a hand on their shoulder can give them heart palpitations. Touching your server is never necessary, no matter how badly you want their attention. As tempting as it may be to reach out and grab your server as they pass by your table, don't do it. An extended finger in their direction or an "excuse me" is 100% better than physically accosting them. Simple eye contact also does the trick.
Your eyes have it.
And speaking of eye contact, not using it is also rather rude. It's not like you need to gaze deeply into the windows of the soul of your server to make a rich connection, but not looking at them at all is downright impolite. Eye contact is how the server / customer dynamic begins and when a customer can't be bothered to look up from their menu or cell phone while the server is talking it sets things off in the wrong direction. You don't have to pull a Tyra Banks and "smize" like a crazy person. You just need to take a second to look at each other and notice that you're both human beings. A little eye contact goes a long way. Conversely, a lot of eye contact goes a really long way, so don't get creepy about it.
This world we are living in is a complicated one and we all have to do our best to make it as livable as possible. Being kind to one another is the best way to do that and if we can pay attention to how we may be inadvertently causing distress for someone else, things will only get better. However, if you are sitting on the bus next to someone playing video slots without using earphones, these people deserve no quarter whatsoever. They're monsters.