Secrets I Found in Undertale

Fighting Sans

I'm in the middle of fighting Sans and I found some interesting things.

What happens if you fight Sans after you spare him:

I know everybody knows what happens when you try to spare Sans but what most people don't know is his reaction the next time you fight him when he tries to spare you since most people skip over his dialogue.
He says:

woah, you look REALLY pissed off...
heheheh...
did I getcha? (with his eye sockets black)
well, if you came back anyway...
i guess that means we never really WERE friends, huh?
heh
don't tell that to the other sans-es, ok?

That's what he says the first time you fight him after sparing him.

What happens from now on if you fight Sans after you spare him:

Every other time you get to the halfway mark of Sans-es fight he says something slightly different than before you spare him. 
He says:

ugh...that being said...
you, uh, really like swinging that thing around, huh?
...
listen.
friendship...
it's really great, right?
let's quit fighting.

This will be his line of dialogue every time you fight him after you spared him.

If you know anything else about Sans-es character feel free to comment :)
I would like to know more paths you can take and things you can learn about the game.
Also, if you would like to see videos of me doing these things tell me in the comment section down below, I would love to make a video for you guys :)