There was a dirty dish in the sink.
That, in and of itself, wouldn't be exactly surprising, seeing as there were no less than eight people permanently living in the Tower, and all using the very same kitchen, because apparently sharing eating space created comradery, but the point of the matter was that there was always some kind of dirty dishes in the sink.
A plate. A mug. A cup. A glass. Forks, knives, spoons – lots and lots of spoons, what could people in this house even be eating that they needed this many spoons – but it was driving Tony insane because someone was leaving all those dirty dishes in there, and he wanted to know who it could be.
See, when they all moved into his awesome tower in New York, Coulson – that bastard who let them think he was actually dead for yearsand then came back, pretending everything was dandy – had suggested that they needed to have some sense of normalcy in their lives if they wanted to keep saving the world without killing each other in between fights. So they had small grievances, like sharing eating space – which didn't bug him as much as he thought it could – and having small tasks they actually had to do themselves, instead of having Tony throw money at people to get them done. It was small stuff, really. Cooking a meal every now and then for the "team" if they went more than a week with no super villain trying to nuke New York. Making their own beds (Tony had needed to get Jarvis to show him some videos about it, because he had a slight problem with an elastic sheet and some pillowcases, but anyway). Going grocery shopping for things that they wanted and weren't generally consumed (like Thor and Jane and pop-tarts).
Doing the dishes.
Of all those things, all the long list of small stuff that Coulson (and Fury, and Maria Hill – and how was that woman giving him orders again, when he was her boss, technically that wasn't fair, ok? At all) suggested they did, doing the dishes had to be the easier one.
Making beds was complicated and a little bit pointless. You were going to go in there and mess it all up again in a matter of hours (well, to be fair, in his case, days, really, but who was counting?), and it was so much work.
Grocery shopping was a freaking nightmare, because once you announced you were going to be out, everyone wanted something special, and you had to get all their stiff, or they'd tell on your unmade bed to Coulson, like freaking children.
But doing the dishes? What the hell was wrong with throwing a little water over it, put it in the damn machine! You didn't even have to turn the freaking thing on – Tony had programmed it to run only when it was full, and it had perfect sensors, thank you very much. It was a matter of seconds.
And yet, there was a whole colonyof plates and mugs and cups and spoons – ALL THE SPOONS – in the sink.
It was driving him insane.
He tried to get Jarvis to tell him who the hell was leaving their dirty stuff around, but the AI just told him he wouldn't – spying on your housemates was not acceptable behavior, apparently. Pepper had installed that particular protocol, and he could override it, he totally could, but he didn't want to cause one more problem he'd have to deal with Pepper, so he gave up on that idea, and decided that some stalking and spying would have to suffice.
Natasha, obviously, wasn't the one being a slob. She cleaned every last piece of used cutlery and even wiped off the counter tops once in a while, because she was just a very neat person. Bruce wasn't it either, and Tony knew this for a fact without having to snoop around, because he had lived here the longest, and there were never any dirty stuff going on in the sink when it was just the two of them.
Thor and Jane definitely weren't the ones either, because of a broken mug at a diner or something. Thor seemed to think that keeping all their mugs cleaned and whole was some sort of acceptable courtship ritual, and that was fine with Tony, really. That guy couldn't be any weirder if he tried, but, hey, Jane loved him. He could do a lot worse.
Sam wouldn't do it on the basis of his military background – he was a lot like Rhodey, really, in that sense. Organized and good at following rules that wouldn't harm civil rights and all that stuff.
Finally, he thought about Clint, but you can't really spy on a spy, so he asked. He got an answer of 'you insane? There's DNA in that stuff, I don't want anyone having access to that, of course I clean my stuff, get off my back, Stark' and he believed him – mostly because Clint was just as paranoid as Natasha and Tony, so it made sense.
Was it him leaving the dirty dishes in the sink and not remembering it? Was that why Jarvis didn't want to tell him, because he didn't want to have to deal with Tony finally having to recognize he was, in fact, insane?
Were the dishes reproducing independently, and then getting down and dirty in the sink?
He was at a loss.
So, the night he gave up on his cause, and thought about just accepting that he lived in a house with a permanent case of dirty dishes, he was in the kitchen, drinking coffee and working on his tablet at an ungodly hour in the morning, because he hadn't gone to bed yet. The sun was just starting to rise when Rogers came in, took a spoon out of the drawer, got a little cup of that natural yogurt only he liked, and started to eat it.
With a spoon.
"Oh my God, IT'S YOU!" Tont roared form his corner, tablet forgotten on the table ad he got up and pointed at Steve.
"What?" Rogers' tone was already offended, as if he couldn't possibly know what he had done wrong.
"It's been you, all this time! The dirty plates and the cups and the mugs, and the spoons!" he pointed at the object in Steve's hands, and the man made to put it down, as if to hide it, but caught himself in time, and just glared instead, "I didn't even think it was possible! You weren't even on my list! You… slob! Dirty dish leaver! All those spoons! How did I not see this before?"
"For Christ sake, Stark, it's just some dishes! I hate doing the dishes, ok? Leave me alone."
"This is just… I just…" and then he started laughing and couldn't really stop.
Steve started out by looking annoyed, and then outraged, and circling back to concern when Tony didn't stop laughing after a full minute.
"You ok?" he finally asked, and Tony shook his head, tears in his eyes from laughing so much.
"It's just – I could make it work with anyone in this house, because everyone had their won hang-ups that I could relate to, but you didn't. It's not that I hated you, but I really, really disliked how… perfectly well-rounded you were."
Steve keeps staring, clearly failing to understand why Tony was smiling about the fact that he didn't like Steve at all.
"But, man, you leave dirty dishes in my sink, and you don't even try to defend yourself. I think I may even like you now."
Steve rolled his eyes, but smiled a bit back, almost fondly – if dirty dishes were what it took to make Stark stop going against everything he proposed just to spite him, starting to do his dishes wouldn't hurt him at all.
"Look, I know Coulson put up all those rules and you guys have all followed through, I'll start doing them, ok? I promise."
Tony was looking at him with a calculating look in his eyes, though.
"What would you say to a trade instead?"
Steve frowned.
"What do you have in mind?"
"My good old Captain, how good are you at making beds?"
(They don't share this story with anyone. At all. When the rest of the team noticed they were getting along better, they said they talked things through as adults.
Tony still doesn't make his own bed, especially now Steve sleeps in it more than Tony does.
Steve has no qualms about leaving all his dirty dishes in the sink.)