Wow, Good to Know! Surprising but important bits of life knowledge everyone should hear. #WowGood2Know

Ever hear something and think, "Why the heck didn't anyone tell me that earlier?" Somehow, people assume you'll pick this stuff up by osmosis before you desperately need to know it. Since that's ridiculous, I'm giving some of my favorite tips straight to my college students (but I'll share them here, too). Thanks to everyone who has contributed ideas; if you have additional suggestions, let me know! (Also, have a look at my "Important Stuff Nobody Thought to Tell You (probably)" page for some similar but longer explorations on a more focused topic.)

Tylenol/acetaminophen OD can be awful: no symptoms for 12 hours, but without help in 8 you need a liver transplant to live.
(It's extra-deadly in combination with alcohol because they're processed by the liver in similar ways. Mixing with caffine may be bad, too. Also, you can get stealth doses from other medicines: Excedrin, Midol, Theraflu, Alka-Seltzer, NyQuil, Vicodin, Percocet, etc. Outside the US, acetaminophen is called paracetamol.) For more information, you might start with Wikipedia or The Straight Dope.
Unless you need help, never talk to police without a lawyer (innocent or not). WATCH THIS: "Don't Talk to Police" on YouTube.
(You wouldn't think that 48 minutes of law-school lecture would be amazingly compelling video, but wow. This should be required viewing in school, especially the first half.)
Racism and sexism are COMPLICATED. Well-meaning people can still hurt others (possibly a lot) out of ignorance.
If you see that your words are upsetting someone, stop talking until you understand why! Social issues like racism and sexism are way more complicated than physics: understanding them requires substantial study and some specialized vocabulary. And just as in physics, some of those terms are common words whose specialized meaning isn't quite what you might think. (Good introductions are hard to find, but this blog entry gives links to some helpful reading to get you started: follow those links! Tangentially related but also worth reading is the satirical Person Paper on Purity in Language by Douglas Hofstadter.)
A tender, warm, swollen spot on a leg could be a blood clot waiting to dislodge and kill you: see a doctor fast!
See Stop The Clot for more info: start with the "DVT/PE Portfolio" section in the middle of the main page.
Don't mix cleaning products! E.g. bleach (like Clorox) + ammonia (in Windex) creates a cloud of deadly gas.
See, for example, this discussion. Other product mixtures can be just as bad, in a whole range of ways.
A dollar saved is two dollars earned: long-term investments double every ~10 years, so start as soon as you can.
(Doubling time depends on interest rate/annual return. For long-term stock market investments, annual return may be 10-11% and double every 7 years. But a simple savings account would take decades.) I'm no expert (so don't take my word for it), but personally I would recommend investing in low-cost index funds.
Bats may have rabies: ~100% deadly. If bitten (or unsure), you must capture it for testing or get vaccinated right away.
No human being in history survived a rabies infection before the vaccine was invented in 1885, and until 2004 every person who developed symptoms died within a week. There have now been maybe six survivors worldwide, using an experimental treatment that works only 10-20% of the time and can still involve some brain damage.
Gender/race/etc. shape life experience, causing not just different events but also very different takes on the same situation.
The simplest illustration I've seen was a high school teacher who asked every student in her class to make a list of the precautions they'd take when walking to their car in a parking lot late at night. The boys kept asking for clarification about why they were supposed to be worried ("Is it a bad part of town?" "Did I see someone creepy?"). Meanwhile, every girl in the class was already well into a long list of standard advice. (An important read on a related topic is "Schrodinger's Rapist: or, a guy's guide to approaching strange women without being maced".)
Be careful with glass in the kitchen: it shatters if the temperature changes too fast or if one side gets much hotter.
This is true even of heat-resistant glassware. Glass that isn't specifically made to be heated may shatter no matter how careful you are about even, gradual heating.
Floss every day! Plaque does its damage after forming a protective "biofilm", which only happens after ~1 day undisturbed.
Some of the details are discussed in this PDF.
Never write computer security code w/out a PhD in crypto: use theirs! Save user passwords with bcrypt; encrypt with GPG, etc.
In other words, when you need cryptography in some application, use well-respected libraries for absolutely as much of the work as possible. Every line of code you write could introduce some subtle vulnerability.
Sleeping <8hrs adds up: "drunk" driving, slow learning, moods, obesity, heart disease, etc. ...and not noticing you're tired.
That's just a sampling of all the bad things that chronically short sleep is known to cause. A long discussion of negative consequences is at this Harvard site, and I also value this striking commentary on how it affects teens.
Only master the office copy machine if that's your job: prioritize to spend time & effort on valuable things only you can do.
Bright/bluish computer/TV screens confuse your biological clock: avoid them for 30 minutes before bed for better sleep.
You can find an overview of recent research on blue light and sleep in this PDF. I've been using the program f.lux, which tries to help a little bit by reducing blue colors at night. Some research on this is discussed on WebMD. I've been using the computer program f.lux, which may help a bit.
Try crazy things (especially while you're young), as long as the risk is less than Penn&Teller's limit: No Permanent Damage.
As one friend said, "Stay in European youth hostels while you are young and poor. Once you get married, your spouse may not care for a bunk bed, single rooms, etc." But the principle is much more general: take advantage of the blanket forgiveness society offers for youthful mistakes!
It's faster to cook a stir fry than to order takeout. Cooking meals at home can be cheaper, healthier, and tastier. And fun!
Phone companies save all your texts: police, etc. can get copies. If used in court, they can even become public.
I can't discuss the circumstances, but in a formal context I was once shown every text message exchanged between two particular numbers over several months. They were very personal; I felt like I was violating an intensely private space. It's probably not reasonable to give up texting, so please, I beg you, take any opportunity to get the laws permitting this changed!
Today's diamond engagement ring "tradition" dates back to a 1938 ad campaign. Prices are kept artificially high by a cartel.
A recent post on the Priceonomics blog gives a good overview. One of its sources is a truly classic Atlantic magazine article from 1982, "Have You Ever Tried to Sell a Diamond?" One key early quote: after huge diamond mines were discovered in South Africa in 1870, the mine owners "feared that when new mines were developed in South Africa, diamonds would become at best only semiprecious gems." So they formed the De Beers corporation to own every significant mine and limit supply. They were successful, and they have for the most part maintained their monopoly even today.
Always be friendly to secretaries, administrative assistants, and anyone else who controls access and budgets.
The entire internet is a public space. Don't do stupid things in public. (Related: Nothing online ever goes away.)
Be very careful arguing religion, politics, or computers with anyone: you'll never change their mind and might make an enemy.
No one on their deathbed ever said, "I wish I'd spent more time at the office." Work is usually a means, not an end.
Tip your waiter/bartender/barber/etc. generously! That extra buck means more to her than it does to you (& respect is good).
12 oz of 4% beer = 5 oz of 10% wine = 1.25 oz of 80 proof vodka = 1.25 oz of 86 proof whiskey = .50 oz of absolute alcohol.
Allow your friends to have at least one major flaw. Allow yourself to have one, too.

Those are already trending in the direction of "general life advice" rather than the "Wow!" factor that I'm really aiming for here. But since people have shared some pretty good suggestions for that, too, I'll include a handful here.

Never be afraid to say "I don't know." Pretending to know when you don't turns out worse in the long run.
Avoid "Geek Social Fallacies"!
Don't pick your pleasure - pick your pain. Deciding early what you can't accept in life puts lesser irritants in perspective.
Wear sunscreen! (and broader life advice)
Travel widely, and live in a variety of places. Don't let your horizons stay forever small.