How well do you really know someone?

I recently asked myself this question,  how well can you know a person? Do you ever stop learning or do you get to a place when you realize you know everything there is to know about them? I asked myself this question because I thought I knew someone but turns out I was way off, and I do mean way off. I realized that I did not know this person at all. This shouldn't be such a shock to me because I'm still trying to figure out who I am. So I really shouldn't be all in somebody else's grill when my grill is all jacked up. So I guess the take home message for me is never think you know someone because they just might surprise ya!