Well... mine isn't exactly a FU story (I think almost everyone said that).
I've been working since I was 19 or 20, I think, so not too young. When I still lived with my parents (up to 26yo, saved some money but not really mustachian, oh had I known!), basically I always considered my internships and jobs from a position of power. After all, I didn't really need the money to live, right?
My first internship was really really nice work, but with a terrible boss, who yelled at everyone and was really unbalanced. My last month there was crazy, she brought 4x more clients than the team had capacity to fulfill, and wouldn't hire new people. In mid-january I told her I was taking more classes that semester and would stop with the internship mid-feb. She asked me to just finish one last thing. I said I might not be able to, but I'd do my best (I did do my best and still wasn't able to finish - it was really a lot of work and I think her way of keeping me there). At least once a week she would casually drop in a conversation that I was staying until the end of that project. I would half smile and give her a serious look of No. Anyway, when classes were about to start, I handed in most things to the other interns and said goodbye. She didn't even turn to look at me when I went into her office. Afterwards she sent an email asking if I could do XYZ for her on the project. I didn't have the balls to say FU, but pretended I never saw it.
My latest FU experience was: I worked as a contractor at this really large multinational company and they merged my team with another in december (meaning they fired everyone from my team). They needed someone to do knowledge transfer to the new team and I was the lowest paid, so they did this amazing offer: you can get your thank-you bonus (2 months salary), work full time for two months (at double my previous PT salary), or work part time for two months (at my previous salary). I laughed out loud when I read this. It was the bonus OR work. So, knowing we can live full well and still save almost 40% of my boyfriend's income, I sat down and wrote a really sweet email where I said this offer made me feel like they didn't value my 3 years of work in the company, and I would love to do the transfer but can't do it only out of the goodness of my heart (these 4 words are actually on my email). In the end I got the bonus, plus full-time salary for 1 month, plus stayed another 4 months in an increased PT salary (because the new office incurred new expenses with transportation and lunch - which of course I banked on by bringing my own from home). And they asked me to stay more time, because I really did love the job and did it pretty well.
Now comes the real FU: for a long time I've been meaning to move to a smaller town, but how? Boyfriend and I passed for this govt job. He was called to fill the position in May, to start in June. If we both got called, we would have this really amazing income that would sum up to almost 100k a year, but no, I wasn't (I was next in line, but they decided they were done and closed the process). So suddenly we would be down to only one income (slightly larger than his previous salary but quite smaller than both of ours combined), and thinking what to do? Well, we have about two year's expenses stashed away, and we can still live comfortably on 50% of his new paycheck. So I kissed my job goodbye, we decided to move anyway and enjoy the new lifestyle, while I look for new opportunities on the new town or telecommuting. In this case, our FU money was the means to achieve a lifestyle choice, not necessarily to quit a job I hated (because I didn't really hate it).
And it's a great feeling to know that whatever money I make will go directly to the stash. I can't seem to see it grow fast enough.