May I Borrow a Cup of Sugar?

A character comes over to borrow a cup of sugar, or something else like flour, a hedge strimmer, or whatever from their neighbour and this is used to set up a plot thread of some sort:

As a Meet Cute
When something is happening really needs keeping secret and neighbor honestly needs to borrow something but has epically bad timing
Similar to the above, but an excuse to barge in their neighbor's business because they suspect something odd is going on and they are a Nosy Neighbor
The borrower has just stumbled onto the home of a serial killer and this is the last we'll see of them alive
To set up the plot in some other way

Please note this is not just a list of instances of neighbous borrowing things in fiction as this has no meaning in itself; it has to at least set up a plot thread or joke of some sort.

Examples of May I Borrow a Cup of Sugar? include:

Advertising

Before he was Giles from Buffy the Vampire Slayer, Anthony Head was best recognized from a series of Nestle Gold Blend (in the UK) / Nescafe Tasters Choice (in the US) commercials about a couple flirting while borrowing coffee from each other.
- Immaculately parodied on Comic Relief one year, where the guy kept trying to direct her to a nearby shop and turned the eventual request down. "Nafcafé Gold Bland. If you like it that much, get your own bloody jar!"
There's an ad for Just For Men hair color where a hot new neighbor chick knocks on our hero's door to borrow a cup of milk and the man says "just a minute," jumps out of his second story window, runs to the store and buys...some Just For Men hair color so he can look young and handsome when he opens the door. He almost forgets to also buy some milk so she can borrow it.
A commercial for Target starts out with a man frantically searching his apartment's kitchen and eventually reveals an attractive woman by his door, holding a measuring cup, saying she'll get the sugar from someone else.

Fan Works

In the last few pages of Drunkard's Walk V, Krishna shows up at the Tarikihogonji temple looking to borrow a cup of sugar from Belldandy, in the punch line to a Brick Joke set up six years and several chapters earlier.

Film

Played for Laughs in the film Johnny Dangerously; Mrs. (Al) Capone comes over to the Dangerously house and asks to borrow a cup of bullets.
Played for laughs in Bad Boys.

Martin Lawrence (in an exaggerated, nasally "Caucasian" voice): We were wondering if we could borrow a cup of brown sugar...?

This literally happened in The Room.
Lucky Number Slevin has Lucy Liu's character introduced to the main character in this fashion. For a little twist, she borrows not only the sugar, but also the cup to carry it in.
In Funny Games, two guys come over to a family's lake house to borrow some eggs. Then they return them broken, and ask for more. It gets downhill from there.

Literature

The True Story Of The Three Little Pigs, a retelling of the classic fairy tail features this. The wolf was only trying to borrow sugar from the three pigs but as he had a bad cold and the pigs had poorly constructed houses he kept sneezing the houses down - and the dead pigs? He's still a wolf, waste not want not.
In the novel Paper Towns by John Green, Margo says she asked a neighbor to borrow a cup of sugar so she could find out more about a man who committed suicide.
There's a Hans Christian Andersen story called "Big Claus and Little Claus" in which Little Claus, having just come into a lot of money, asks his neighbor Big Claus for a bushel to measure it with. Big Claus, curious about what Little Claus is measuring, smears tar on the bottom so that some of it will stick, and is astonished to find three silver coins sticking to the measure when he gets it back.

Live Action Television

Alice Kravitz uses the Nosy Neighbor excuse version fairly frequently in Bewitched.
In Not Going Out this appears to be a Meet Cute, but turns out to be the excuse version of sorts when a girl who thinks she is Lee's daughter appears, of she isn't its a sitcom. When a guy shows up at the end of the episode asking to borrow some milk Lee tells him there's a Tesco nearby and shuts the door on him.
On an episode of The Young Ones when they are freezing in their unheated house in winter, Vivien keeps borrowing cups of sugar from the neighbour, who comments that she would like her cups back at some point. It turns out Vivien is burning the cups to keep warm.
This would often be a way for Lucy and Ethel to meet on I Love Lucy.
Parodied in the Gilmore Girls episode "Luke Can See Her Face"

Luke: The guys next door just ran out of crack to sell so they sent me over to borrow a cup.

Star Trek: Deep Space Nine episode "When It Rains..."

Dr. Julian Bashir: I need to borrow... a cup... of goo.
Odo: Excuse me?
Dr. Julian Bashir: Please? I'll give it back.

Captain Power and the Soldiers of the Future episode "The Intruder"

Andy Jackson: Hi. Just, eh, dropped in to borrow a cup of sugar.

On The Dick Van Dyke Show Millie would often pop over to borrow a cup of sugar from Laura. Laura was more likely to call Millie to ask if she had some sugar that she could borrow, making Millie come to her.
In To the Manor Born, Mrs. Polouvicka uses borrowing a cup of sugar as an excuse to meet her new neighbor—who, unknown to her, is none other than Audrey fforbes-Hamilton. At the end of their encounter, she confesses that she didn't need the sugar, and tips it back into Audrey's bowl with a grandiloquent "Keep it -- with the compliments of Cavendish Foods!"
The Loquitos Sketch in Chespirito had a neighbor of Lucas and Chaparron who was constantly asking them for a cup of sugar.
Mystery Science Theater 3000: Pod People. Little Tommy walks into a cave that's glowing bright red, while holding the cup he normally uses to collect specimens. Crow jumps to conclusions:

Crow: What is it about the gate of Hell that always compels people to wander into them? ... What's he going to do, borrow a cup of sugar from Satan?

In the pilot of Desperate Housewives, Susan uses this as an excuse to break into Edie's house to spy on her and Mike. She ends up (accidentally) burning the house down, and leaving the cup behind.

Radio

The Jack Benny Program, like many sitcoms, had a Running Gag of the Unsympathetic Comedy Protagonist borrowing things from his long-suffering neighbor (in this case, Ronald Colman) and not returning them. In the March 19, 1948 broadcast, Jack borrows Colman's Oscar award; when Colman's wife asks why he let Jack borrow it, he snaps, "It might as well be with the rest of my things!" As Jack walks home, the Oscar is stolen by a mugger in probably the most famous bit in the program's history.

Yosemite Sam: [interrupted by a knock on the door] Ooh, what do you want?
Bugs Bunny: Good afternoon. Let me introduce myself as one of the woodland creatures residing in the King's forest, and being that there is such a close relationship, I assume it would not be too improper to borrow a cup of diced carrots, huh? [he holds an empty cup]

- In Duck, Rabbit, Duck!, Daffy asks Bugs for a cup of blackstrap molasses as an excuse for Bugs to come out and get shot by Elmer. The shot punctures the cup instead, and Bugs comments that "I didn't think molasses would flow in January."
On one Donald Duck cartoon, Donalds new neighbor Pete asks to borrow some ice cubes... and while he's at it, he also borrows a cube of butter, some cucumbers, and on and on until he empties Don's fridge.
Family Guy - Mort has borrowed so many things from the Griffins without returning them that Peter once put an Adolf Hitler scarecrow in the yard.
On an episode of SpongeBob SquarePants, SpongeBob wants Squidward to come visit, but Squidward doesn't want to. So SpongeBob borrows his vacuum to give Squidward an excuse to go over to SpongeBob's house.

Web Animation

Homestar Runner - Strongbad_email.exe: Disc Five

Marzipan: Hi, Strong Bad. This is your next-door neighbor the crocodile. Can I borrow a cup of sugar? I'm going to make some puppy-dough cookies tonight...