Is it common for guys to act like he is into her and then later told her we're just friend?

My take on it:

Most "nice guys" usually get tired of constantly ending up in the friend zone when they're in their early 20s. So when a guy reaches that point, and starts to seriously care about himself rather than being nice to girls, he sees that the power landscape is shifting.

From my observations, at this stage in life men grow more confident, while women usually grow less confident ("the 20-year crisis").

This means:
1) The guy wants to see if he's still "got it", and therefore flirts you up without wanting anything else.
2) The guy just wants to be nice. From past experiences, he would think that being nice, casual and giving compliments would be a way to play it safe (in the sense that he doesn't want anything from you). Little does he know that many girls in their mid- to late 20's are actually looking for a "nice guy" after having their share of experiences with "assholes". And when he finds out, he doesn't care - it's nothing wrong about being nice, is there?

I believe it's a common misconception that guys are always looking for that one thing. When the game suddenly tips in our favor, we can be nice, flirty, funny or whatever without necessarily wanting sex.