What’s an unspoken rule that absolutely everyone should know, but most people clearly don’t?

Give LOTS of space when passing a bike in your car.

Give EVEN MORE when passing a horse.

Don't make phone calls on speaker when in public. Not even if you hold it up to your ear.

Do NOT stop to talk in doorways.

If you're boarding a train or bus let people exit first instead of squeezing your fat ass past everyone to get a good seat

Let the people out of the elevator that are trying to get out of the elevator before you attempt getting in.

When driving, you should engage your turn signal before you start applying the brakes in preparation for your turn.

Here's a weird one:

Don't offer advice unless its something you have some experience with.

Googling someone's issues and giving them a boilerplate answer from the first thing you find isn't helpful and can actually be a hinderance more than anything.

Don't stop in doorways when walking through public places.

Elevator Etiquette: Let people exit before you enter. That way you have more room to enter. This applies to all things, not just elevators.

When it’s close to closing time, get out of the store. If you have items you want to buy, leave yourself enough time to check out before the store closes. If you’re bringing a full cart to the register 30 seconds before closing time, you’re an asshole.

The employees have probably been there for 7+ hours, and still have to clean, restock, etc. They don’t want to be there for one second longer than necessary. Don’t be the reason they get home even later than usual.

Be aware of your surroundings. This is not just a safety thing for yourself, but a courtesy thing for other people.

Put your cart back when you're done shopping.

If you need to stop while walking on a path, inside or outside, step to the side. I can’t believe how many people just abruptly stop to look at their phone, have a conversation, point out something they find interesting… like my guy, would you do that in a car? Fucking GOOOOOO!!

Since the majority of people in the world are idiots, and management tends to collect them at a higher rate than other roles, you will generally be rewarded more for how you make others feel than the amount or quality of your work. A person with a 10 in charisma and 4 int gets farther than the person with a 10 int and a 7 in charisma most of the time. Focus on learning better people skills. If you find a place where your work is recognized you should cherish it.

Also in the US the left lane is for passing the right lanes.

Zipper merging.

If your lane is closing ahead, it is better for everyone in traffic if you drive all the way to the end of the lane and cut in at the last moment.

Note that this does not apply to exit lanes. The basic rule is if late merging blocks someone from going somewhere, merge early. Otherwise, merge as late as you can.

Do not start or continue a phone conversation when going to the bathroom - public or private restroom but especially in public bathrooms.

You stand on the right side of an escalator/moving walkway. You walk on the left side.

It doesn't matter if you have multiple people or luggage, the right is for standing, the left is for walking.

Push your chair in after leaving a table at a cafeteria. It blocks the path for people who are carrying trays and may not have a free hand to push it in themselves.

When shopping and it's busy, don't walk down the middle of the aisle or leave your shopping kart there.

Zipper merging

When waiting at a red light, cars are supposed to stay out of the pedestrian crosswalk

People exiting a building or vehicle get priority over people entering. Let people out before you go cram yourself in the way.

If you're in a drive on the left or right side of the road country, that goes for bike and walking paths too. Eg in Australia, keep left on footpaths.

Even if you have a green light at the crossrowds you shouldn't drive if you can see that you have no way of exiting the crossroads and would block others e.g. there is a traffic jam on the road you want to drive into.

(at least in Poland)

There's really only two of them:

1. Don't be stupid.
2. Don't be a dick.

They're not even unspoken, people say them all the time, but some people just don't pay attention I guess.

Remember to be kind.

Reading the answers to this questions, I'd say "Not understanding the meaning of most people".

When exiting an airplane, it is more efficient to remain in your row until the row ahead of you has accessed their belongings. This includes people that have no overhead luggage. Sometimes someone's overhead luggage is behind their seat and it causes noticeable congestion/delays if there are people standing in the aisle obstructing their path. An exception could be made for patrons that need extra time for mobility issues, but this is usually arranged in advance with the flight attendants. Having said that, it's best to make every effort to exit with expediency because there could be others that have very little time to reach their connecting flight.

It's safe to generally assume that most others are equally (or more) tired of being on the plane and want to leave ASAP.

don't hold your phone while driving

"Do unto others..."

Even those who supposedly devote their lives to such teachings don't seem to know wtf that means

If your seat reclines in an airplane, you have every right to recline it except during takeoff and landing. This one is controversial.

Don't be a dick.

This is probably just a me thing, but if you have to use the restroom, just say that instead of saying what you specifically have to do. I don't need to know any details and you don't need to share them with anyone. The only details worth sharing is if something happens, like a pipe leak/burst or something else serious.

One could write a book on unspoken rules for tipping in the US. Do you tip on takeout? Do you tip baristas? Does it depend on the beverage? Do you tip if you sit down but bus your own table? What's considered a good tip (and this is situation-dependent)? The only thing I can tell is if you're worried about something happening to you, then 20% of the price is generally a safe amount.