The rambling lecture that your headmaster gives you will actually be really, really important later on — e.g., "Now pay attention, Potter. This bird can carry huge weights, is naturally loyal, its tears heal wounds, and by pure coincidence its tail feathers went into your wand and your mortal enemies'. I wonder what use you could put it to, hmm?"
Isn't the real Aesop then "pay attention in class, 'cause a few items mentioned at some point during an entire school year might save your life"?
It's a good idea for an abused child to follow a seemingly kind stranger who then brainwashes the child into being part of a cult where people believe they can make "magic" with sticks.
When all your efforts towards a goal fail (not that you should try really hard in the first place), apply a mind-altering substance — it will give you all the correct answers, direct your actions along the only right path (don't bother to question either of those) and will even alter reality for your good.
Give children weapons that can kill/torture/maim people instantaneously. Just tell them not to. It'll probably be fine.
If a schoolyard brawl does happen to result in the maiming of a fellow student at your hand, then you can expect several hours of detention, causing you to, at worst, miss an intramural sports match. Your team wins the cup in your absence anyway, however, and you can still share in the celebrations, including a public make-out with your best friend's younger sister. The whole episode of the maiming is soon forgotten.
Loving your family is wonderful, unless you report an illegal secret society to the proper authorities in order to protect them. If you do that, you deserve to walk around with a disfigured face for the rest of your life.
Hey girls! The guy who sexually humiliates your best friend in front of the entire school to blackmail you into dating him will be an excellent husband to you.
Journalists are jerks. Except those who believe in all conspiracy theories.
While we're at it: a bastard journalist you blackmailed is more reliable to transcript a story you dictate word by word than writing it by yourself.
Mind-controlling and torturing people is wrong, unless you're the hero.
You Guys! You're forgetting the most important one: 'It's easy to become a maniacal dictator who throws the world (i.e. England) into a Crapsack spiral. Kill anyone who defies you! However...you should probably stop at one-year-olds and loving mothers. They can, like, totally ruin your plans...by dying...and being a lovable infant...'
If the relationship between two pupils under your charge has deteriorated to the point that they're trying to seriously kill each other, resulting in a fight to the death that leaves one maimed, the most important lesson to teach the other is to remind him just how much you still hate his dead father.
If your specially-designed weapon for slicing people open is used by a student, it's the perfect time to go on about how horrible his father was.
If the person you used said weapon on was an enemy of yours, don't bother feeling too bad for using it. Even if he was having a nervous breakdown at the time you used it on him. And don't bother offering him your help while he recuperates. He was mean to you! He doesn't deserve your pity.
When you are told that some acts are so evil that doing it once result in life sentence in prison, they never mean it. Actually you can do it every single day, it's okay if you do it for greater good. And even if you do it for evil, it will be forgiven if you switch side at the last second. this refers to Dumbledore forgiving Draco, even if he used Imperium for a whole year.
Warping the warp: Go ahead and abandon a juvenile delinquent for an entire year. If you offer him a chance at redemption when it's too late for him to take it, you'll still go down in history as a good person, not a manipulative bastard who kept crucial information to yourself.
It's better to leave a child in an abusive home than to run the risk that he'll grow up to be proud, spoiled, or egotistical.
Because it's not like kids in abusive homes are people, prone to the same character flaws as the rest of us. It's completely impossible for a child in an abusive home to become negatively warped by the experience. Nope, every kid in an abusive home is an absolute saint.
Hey! A boy has a scar that gives him horrible pain? It's best to not remove it even if you knew how. Because your selfishness far outweighs a boy's comfort of not feeling like his own head is on fire and about to crack open from the inside.
Relationships that began when you were seventeen will always work out so that you are happy with plenty of children in twenty years. Also, your friends will never change, in fact it will be like the twenty years never happened except for said adorable kids.
Alternatively, if you're an author writing a Distant Finale, you better make sure your characters' friendships and romances don't break apart during the Time Skip or your fans will go apeshit and a coin a new termnote "getting rolfed", named after Luna Lovegood's offscreen husband in the finale for your treachery of daring to give your characters further development off-screen.
You should never be afraid to underestimate your enemies. Being evil will strip them of the very last shred of reason, imagination and the ability to learn from their mistakes.
School bullies and his/her friends will never, ever, not even when their lives are in chronic peril by some homicidal loon, decide to do the right thing and help out. They'll always try to sell out the hero. Therefore, it's okay to kick them out of the school without a second thought.
If you are an unpleasant female character than you deserve to be ravaged by centaurs. However if you are an attractive male villain you can get away with mind control and attempted murder and everyone will forgive you.
It's for the best for a young, impressionable child to be kidnapped by a secret cult society that's still stuck in the middle ages and be sent to a dubious "school" that doesn't even teach basic subjects like Math, Science, P.E., English, foreign languages... (unless you count severely mangled Latin.)
If the Big Bad is a result of you being a sanctimonious bastard and leaving a child with "dark tendencies" in an abusive household then the obvious solution is to do it again to another kid.
Stalking and acting like an entitled nice guy bastard makes you a good person, your creepy "love" for the woman you got killed will be seen as somehow redeeming of your overall nastiness to the point that the boy you abused for years will even name his kid after you.
It's perfectly health for a society to be so isolationist they don't have the slightest idea of what basic technologies or clothing rules exist in the outside culture. In fact, play it for comedy: it's not creepy, it's funny!
Harry Potter and the Prisoner of Azkaban:There's nothing wrong with accepting an insanely expensive vehicle as a gift from an anonymous stranger, especially when you have already pissed off some of the most powerful people around, and you know that such vehicles can be rigged to kill their driver. Those of your friends who treat such gifts with caution are just narks and deserve to be shunned.
Harry Potter and the Order of the Phoenix: So, you've got an adventurous and independent child who also happens to be a trouble magnet, has a murderous supervillain after him, and has been known to act without thinking? Don't tell him any relevant details, ever. It's not like he'll try to find out on his own and end up getting into even more trouble along the way.