I (F, now 22) used to be acquainted with another girl my age (we were both about 17 at the time), it was a typical friend-of-a-friend situation. I am bisexual and thought she was really cute, but never once thought that she could possibly be into girls as well, least of all me.
My self esteem was extremely low around that age and I did all kinds of mental gymnastics to explain away all of her flirting. I once saw her at the grocery store and she became really shy and didn't approach me (she was very shy in general) but later that day sent me a message saying something like "I saw you at the grocery store today, you looked cute".
But the absolute boldest move I completely ignored was Valentine's day when we hung out as a group with our friends and she walked over to me and gifted me a hint so broad, I can't explain how the wrapper didn't burst: She had made me handmade heart-shaped chocolates and put them in a small transparent gift bag. There was only one other thing in the gift bag, the only thing in the universe that could be gayer than heart-shaped chocolate from one gal to another: a little paper card with some kind words on it and printed on the other side was a picture (a manga panel from the looks of it) of two girls holding hands.
And my only thought was: "Wow, what a nice girl! I have to be really careful not to develop a huge crush on her, since she couldn't possibly be into me! She probably felt bad for me on Valentine's day because I'm so single and unfortunate-looking. Those straight girls really lead you on without meaning to!"
Today I looked back on this situation with horrified realization, as Facebook told me she got married recently. To a woman.
TL;DR: I thought a teenage friend had just been REALLY nice to me, realized she had been gay the whole time when she got married.