Two pretty unlikely cases.
The first one is pretty stupid on my part. When I was 17 I visited a school I had basically no interest in. In the afternoons, when everyone went home, I got out of class almost as if someone had woken me from a week-long coma. I was always insanely tired and bored. And one day, on the way out, I tripped on a rather big set of stairs because I was so god damn tired. It just so happens that my soon-to-be friend (basically your typical 'popular girl') was talking to her friends on the end of that set of stairs.
So here I am. The total creep with long hair and a black textile coat that everyone tries to get away from, flying down a set of stairs in the general direction of some girl while her friends started screaming as if a comet is about to strike our planet out of existence. Well, what can I say: She actually caught me. We still fell to our asses, but she made sure I got out of it without seriously injuring myself. (Or her). Things got going from there and we're great friends to this day.
The second one is a bit more typical. We all know the case of the spilled drink in someones backpack right? Well, in 5th grade, one of the guys from my class had just that problem. All his papers where pretty much useless, but worse yet: The guy was thirsty as all hell because we just came from sport. I offered him my water and we got talking right away. We walked the whole way back to our school together and that pretty much sealed the deal. Over the next few months we became friends.