Hindsight is 20/20, and it's really easy to see where you went wrong after the fact. Sometimes it's nice to try to save someone else from making the same mistakes you did.
Unfortunately, a lot of things don't seem like the good advice they are until after you've already messed up.
Reddit user u/B00FI asked:
Keep a close eye on your relationship with substances. Many of you will have serious problems with them and won't realize until you're in very deep.
Triple redundancy...back up ALL your photos, videos etc, I lost about a year of photos which included a couple of big life events, mostly I backed up but got sloppy.
Cloud storage is very cheap but don't rely on just that, put it on a hard drive as well and a few other places.
Also for those who are a little older check all those old DVD/CDs you burnt as they're degrading and data is being lost.
Always watch how your love interest treats other people. The day will inevitably come when that person will treat you the same way.
Not breaking up with a guy immediately after he hits you for the first time. I'm sorry doesn't press the reset button.
Credit cards are not free money. You may need one to build credit but be careful how much you use it, and pay it off every month. It's so easy to charge "just this one little thing" over and over until the card is maxed out.
If your grandparents/older relatives are alive and well, spend some time with them, call them to say hi, just check in with them. When they are gone you're going to be the one left with regret, not them.
Avoid staying in a job where you're totally bored or hate facing it every day.
Some people stick with it endlessly because "it takes too much effort to find something new" (or because they believe an alternative will probably be no better in the long run than where they are).
When you have your first well-paying job save some money. There's taxes you'll have to pay. Plus you might not earn that much forever. Save money even if you think you don't need to. I'm 21 and broke (in debt) even though I earned like 70k a year ago.
Do not lie to your S/O. They're supposed to be your best friend. If you lie, trust will break down, the relationship will start to suck, and then you will end it due to their seemingly unwarranted paranoia, or they will end it because they cannot trust you.
When picking a career it's best to focus on what you're actually good at than something that just sounds more interesting. Regrets have been made.