Never Lend to a Friend

Neither a borrower nor a lender be
For loan oft loses both itself and friend,
And borrowing dulls the edge of husbandry.

—Polonius, Hamlet

It's coming up to rent day and Alice is a little short. Bob, on the other hand, has money to spare, and casually offers to lend her the cash - after all, what's a few quid between friends? No need for collateral or payment deadlines, The Power of Trust will prevail!

Now one (or more) of several things happens to severely strain their friendship:

Alice is slow to pay the money back. Bob starts off understanding, but gets more and more annoyed the longer it goes on and the more his own financial pressures build up.
- Bob assumes this will happen and starts pestering Alice for the money unreasonably - she's already paid it out, and can't return it yet, and her good friend has suddenly transmogrified into the All Devouring Black Hole Loan Shark!
Alice suddenly has money to burn, and does so. Bob is either angry that she hasn't learnt her lesson, that she seems ungrateful, or that she apparently didn't need the money as much as he thought. He may be too proud to mention needing the money, and Alice will assume he's getting along fine without it.
- Bob assumes this will happen, and starts watching over Alice's shoulder, reading her receipts, and generally being nosey and superior - less Loan Shark and more Bank Manager.

This will rarely be the moral grey area it would usually be in Real Life: one party (most often the greedy lender) will be clearly in the wrong, and will learn An Aesop about the importance of Friendship and Trust. If the borrower really was taking advantage, the relationship can be more significantly damaged, especially if they continue to refuse to pay it back.

One of the Money Tropes. Closely related to Broken Treasure, where a borrowed possession is lost or broken, leading to similar problems. If the ill-advised borrowing is from a suspiciously helpful stranger, they will probably turn out to be All Devouring Black Hole Loan Sharks.

Examples of Never Lend to a Friend include:

Anime

Cosigning a car lease for an old work acquaintance is what sets the events of Kaiji into action. And, of course, the guy he did it for shows up and proceeds to die, ensuring Kaiji will never get re-payed for the headache.

Comics

In Knights of the Dinner Table, a Running Gag is for one of the characters, usually Dave or Bob, to show up to the game with some expensive extravagance, like a $75 electronic ~~GM screen~~ "player advantage screen", or drop everything to spend a week at GaryCon, with long-suffering B.A. or Sara pointing out that he still owes money or that his car has urgent repair needs he's been putting off.
- One strip deals with all five characters dealing with an tangled web of World War I alliance-proportions' worth of owed money between them. The equally complex solution ("Take the money you owe me, pay it back to him", etc.) clears up everyone's accounts except for Bob, who now owes money to everybody.

Film

Subverted in A Bronx Tale. Calogero is owed twenty dollars by a casual acquaintance, which escalates into such a tense issue that the kid ends up just running every time he sees "C" coming. C is venting about this one day to his men tor Sonny, who asks C if he even particularly liked the guy to begin with. C replies that he never really did, and Sonny points out that he's free to just forget about it if he wants—the other kid will continue avoiding C in order to avoid repaying the debt. "He's out of your life for twenty dollars."

Literature

In Guy de Maupassant's "The Necklace", a woman borrows a fancy necklace, loses it, can't bring herself to tell her friend, beggars herself and her husband to buy an identical necklace to give back... and then, after a lifetime of misery based on that single decision, encounters the lender again, spills her guts, and discovers that the woman had only lent her costume jewelry.
In George Eliot's Middlemarch, Fred Vincy casually persuades Mr. Garth to underwrite a debt, assuming that he will easily pay it back from an expected inheritance. When this doesn't work out as expected, he tries to scrape up the money owed but comes short, forcing the Garths to give up their life's savings which were earmarked to fund their children's apprenticeship. Fred is guilt-torn, but later, when Mr. Garth's fortunes improve, it's he who gives Fred the means to redeem himself and repay the money.
In one of Lewis Carroll's Sylvie and Bruno books, the Professor tries to explain the meaning of the word 'convenient' with a poem about two men, Peter and Paul, which begins with one deciding as a gesture of friendship to lend the other fifty pounds. Said poem takes this trope to its extreme, as the lender does not find it "convenient" to provide the money until well after the date in which the lendee is forced to pay it back... and after the lendee is reduced to homelessness... and while the lender still hasn't found it convenient to lend the original money at the end of the poem, he has decided in his magnaminosity to lend fifty more pounds! Which the lendee rejects, exclaiming that "it would not be convenient!"
In one story by Ephraim Kishon. Played with insofar as it's the friend who really becomes obnoxious, despite the narrator being polite and helpful.

Live Action TV

The Big Bang Theory: Inverted: Sheldon honestly doesn't care about the money he lends Penny, but she gets neurotic about it.
Everybody Loves Raymond: Ray & Debra lend Robert money after visiting his run-down bachelor pad, but Ray gets upset when Robert goes to Las Vegas.
Frasier: Frasier lends Roz some money to help her through single motherhood, but calls her spending into question when he sees luxury items in her shopping bag. Turns out they were all justified expenses (a gift from her mother, a store credit for a return, etc.) apart from one (a bottle of perfume) which she got to treat herself.
Twisted in House, where House asks Wilson to borrow money any time he makes a big purchase. He actually has the money, he's just trying to objectively measure the strength of their friendship.
iCarly devotes an entire episode to Sam paying back Carly and Freddie $500. Sam ends up getting a bad job and it strains their relationship somewhat.
Sex and the City - Carrie needs to get a mortgage on her apartment, but has apparently managed to spend all her money on shoes (no, really) so she doesn't have it. Miranda and Samantha offer to loan her the cash (she refuses) but Charlotte doesn't, because of this trope. Carrie whines about it, and Charlotte eventually changes her mind and lends the money to Carrie, who promises to pay it back with interest. It's never mentioned or brought up again.
On Cheers, Diane borrows $500 from Sam to buy a first-edition Hemingway. Sam says he's not going to expect her to pay it back, but then Carla eggs him on by pointing out Diane's expensive clothes, lunches, etc. Finally Diane gives Sam the book as collateral; he drops it in the bathtub while reading it. A buyer offers Diane $1200 for the book, and Sam is forced to outbid him.
- Another Cheers example: Norm suddenly comes into money and Sam starts harping on him about his bar tab. When Norm buys a boat with the money, Sam loses it and starts yelling at Norm. Norm reveals that the boat is for Sam for being such a good and patient friend.
Several examples in Friends:

Joey: If you want, I could loan you some money?
Phoebe: Oh no, no, no. I learned never to borrow money from friends. No, that's why Richard Dreyfuss and I don't speak anymore.

- And:

Joey: You know... lending friends money is always a mistake.
Monica: But Chandler lent you money!
Joey: And I think he would tell you it was a mistake.

- While in an inversion, Chandler spends an episode desperately trying to lend Joey money, but Joey doesn't want to owe him anything - presumably because of this trope. And thus was born the Game of Cups.
The Suite Life of Zack and Cody: Maddie borrows money from London, and London uses this to guilt Maddie into doing things for her. In the end, Esteban and the rest of the staff take up a collection so Maddie can pay London back.
Partly Lampshaded and partly averted in an episode of Doogie Howser, M.D.. Vinnie asks Doogie for some money, and at first Doogie refuses because of this trope. He ends up agreeing, but it doesn't cause any problems between them and it never gets mentioned again.
A LOT of the cases on Judge Judy involve the plaintiff suing a former friend for an unpaid loan. The defendant's usual defense will be "it was a gift, not a loan," such as in this case. Judge Judy almost always rules in favor of the plaintiff, as well as giving them the advice: "Never lend money to anybody. As soon as you lend money, you become the bad guy."
In an episode of M*A*S*H, Winchester loans money to BJ, then proceeds to treat him like a servant, expecting him to do everything he wants. For some reason, BJ complies, even though he already has the money and these conditions were never discussed when he asked for the loan.
- Another episode has Frank and Hot Lips arguing over this, including the obligatory mention of the page quote.
JD on Scrubs lent the Janitor a buck for the vending machine, only for him to start acting as if every random encounter is JD hounding him for a repayment. Of course, that's how he acts most of the time with no provocation whatsoever, so...

Music

"Listen Up" by The Gossip:

Everybody knows someone like that
Who borrows money and won't pay you back
They'll talk about you at the drop of a hat
Lie about it to your face when they're caught

Theater

The page quote from Hamlet was in the middle of a bunch of other, nonsensical, advice Polonius gave his son Laertes, implying that it was also not great advice.

Real Life

Real Life: some religions ban money-lending with interest entirely, as it can be seen as capitalising on another's misfortune ("Usury"). This is the case with Islam, and was also true in Medieval Christianity, with the interesting side effect that people simply borrowed from the comparatively Unfettered Jews, giving rise to the "greedy Jewish moneylender" stereotype that unfortunately survives today.
- The webcomic Suicide for Hire had a field day with this one, when beating up a bunch of Straw Man Christians
- Judaism includes the same ban, but the Jews were able to charge interest because of a bit of canny Loophole Abuse: they weren't allowed to charge interest to fellow Jews, but Christians were on their own.
It's worth noting how many broken friendships are caused by this. In fact, there's a reason why a gigantic chunk of cases on most "judge" TV shoes have to do with loaning money. It's better to treat any money loaned to friends as a gift, although it would be nice if they paid you back.
- For the majority of these cases, the agreement was only verbal and it gets hard to prove that the friend owed the other friend money when one can easily "forget" or not remember the details of the loan correctly.