Not 100% hackathon, but I was once in one of those weekend coding challenges - aka: have idea, implement MVP, present to a Juri and get a chance to win a prize.

So, to start things off, you had a few months to prepare the idea, gather a team (minimum of 2, maximum of 5 per team) and register.

I gathered a few friends from university, that was cool. We were 5, I had the idea already, they agreed. I started talking business with some partners/governmental stuff (no time to explain all, ask in comments if you want to know).

2 weeks pass by after registering, still 1+ month before the event, 2 of the team members let me know they want to focus on university, so they cannot spend a weekend on this competition. Well, ok, still 3 people, no worries.

Fast forward, 1 week before the competition, another one says he won't be in town, we're 2. Still enough, we meet the requirements, it's just for the fun anyways.

Day 1 of the competition, I'm there waiting for my other teammate. Call him countless times, doesn't pick up. Later tells me he's sick.

I tell the organization about it. They asked: You can continue, but it's fine if you give up now.
> Yo, dafuck you mean give up? I'll die before I give up. It's for the fun anyways, worst case scenario I spend a nice weekend doing what I like *shrug*

So there I am, all alone, doing a first MVP of the mobile app in Android (without any prior android experience, and don't ask me why I chose to do mobile app for that project, was stupid back then).

Lots of nice things there, overall a good weekend, networking, food, gadgets and stuff like that.

Juri day, put on pretty clothes to present my super idea alongside my super MVP of the ugliest mobile app I've seen.

Judge 1: likes the idea, ugly app.
Judge 2: likes the idea, ugly app, could improve and work on the concept, etc
Judge 3: Lots of business questions, to which I came prepared with already potential clients and partners, liked that part although seemed a little confident of it working or not.
Judge 4: "Yo, that's the most stupid thing I've heard, not even gonna ask questions, that's just stupid"
Judge 5: A teacher in my university, the one to actually tell me about this competition, kind of like that meme from "How to train your dragon" where he does the thumbs up thing. Obviously the app sucks, but understandable, no one in the competition has much experience, bla bla bla

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Final decision: No prize, fuck the idea, got a participation amazon voucher of like, $10 usd. *shurg*

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Fast forward a few months, my aunt who shared the idea with me and who i was working with before the competition, sends me a link for an article on FB messenger.
The company where that MF judge worked at build a system exactly like the one I presented, claiming it was a very innovative idea. Never heard of them again, it was a consultation company (Deloitte), so I assume they didn't sell it well and dropped it also.

Moral of the story: I guess there's no moral, just have fun.