A few times have stood out over the years for me.
In first grade I used to have three (yeah, three) different girls take turns laying their head on my shoulder during story time. I even had a crush (granted, a first grade crush) on one of them. And what did I do? Violently shrugged my shoulders to knock them off. Because I thought it was funny. Ironically, I would later learn that one of them (not the one I had a crush on) had a crush on me through the entirety of school, all the way through senior year.
Then in middle school, I had a crush on a different girl. We were in the percussion section in band together, and talked all the time, but I'm paranoid and was somehow convinced she had a crush on one of my best friends, despite showing no signs of that actually being true. So naturally, me being an idiot, I tried to set them up. I told her that my friend liked her, although she never actually responded, so I was never really sure if she heard me. But towards the end of the year, she mentioned that she was going to transfer to the trumpet section in band. Where the best friend was. Strangely, enough, they never seemed to get together. They did flirt a lot, or at least it looked like it to me.
There was also a time in that same year that I got a call from a girl who asked me if I wanted to go out. I was friends with her, so again being an idiot, I assumed it was some kind of prank (in my defense, I heard laughing in the background). She insisted it wasn't, and I somehow mumbled an "I guess" before hanging up. The next day she was telling everyone we were going out, which I vehemently denied, still thinking it was a prank.
Later in high school, I would have a few classes with the girl I had a crush on in middle school. In driver's ed we got paired up along with another for some driving practice. We talked pretty much the whole time, and she held onto my arm while the other person was driving. Didn't really think anything of it. She mentioned after the fact that she really liked being in the same group, and when I told her we were in different groups the next time, she seemed pretty disappointed. Once again, me being me, I did nothing about it.
We would later be in the same German class together, and we would frequently talk and pair up together for assignments. She also sat right in front of me, and when I would lay my head down on my desk, she would always lay her had back and turn towards me, so we would be like 4 inches apart. Again, I thought nothing of it. Sadly, she later started pairing up with someone else for assignments until we eventually barely ever talked. I learned next semester that she was dating the person she had been pairing up with. Never spoke with her again. I did see her on graduation day and tried to smile at her, but she either didn't see me or was ignoring me. I still don't know if she ever actually liked me or not. The signals seemed very mixed, at least to me, and I'm still not convinced that the signals I thought I saw weren't just in my head.
There was another minor instance of a girl at my job. We worked in the toy department at Walmart, and we were rarely too busy, so we would always talk. Looking back she did laugh an awful lot, and when my sister came in to shop, immediately (and almost accusingly) asked me if she was my girlfriend. Dunno if there was something there either, but she quit like a month later anyway.
In short, I got no game. Like many, I have problems figuring out if someone is flirting or just being overly nice.