The thing is, I accepted an invitation. I didn't even know I did it. I was simply clicking around, learning how this site works. There wasn't an intentional invitation I made that I wanted to cancel. I simply didn't know. Now I do. Isn't there some grace factor? To me, that is sporty. Saying, you clicked it, tough is a bit strict in my opinion.

It would be similar if we bumped into each other on the street while being preoccupied with whatever we were doing beforehand. You juse say, "excuse me sir" / cancel, and move on. Not only do I see that as sporty, but also polite.