Of course this is inspired by the question from the application for YC but I'm not applying this year and I'm genuinely interested in sharing and hearing some other great anecdotes.
For me, I think it was when I used social engineering to track down the programmers who designed a very expensive product for a vendor in my industry and then negotiated a price to build the same full featured product for 1/5th of what the vendor charges for a one-off job. They were freelancing on odesk/elance and I had used the demos that vendors offer of their products to categorize the features and estimate what technologies were involved. When I talked with the programmer he admitted about how much he was paid for work on their product.
The industry is e-discovery for law firms, btw.
"Before christmas, I was buying gifts for my young cousins and I found out that Toys"R"Us had a policy where they would match any competitor price. I used this to my advantage, I was able to get presents, using my iPhone, that were being retailed at $40.00 for around $8.00. Other shoppers asked me to show them how to do the same. I helped several people free of charge. Toys"R"Us was not fond of it, I just liked helping families out and showing them how much money they were able to save."