Because the report buttons are fairly small, and it isn't too hard to derp-click the report button on the champ just above/below the intended target. I try to remember to check and make sure it is the right person after I click, but sometimes I forget to, and only realize it when the honor button is gone on a friendly champ :p
I jsut hate the thought of that happening. I've accidentally reported friends who I queued with before lol. Maybe I'm just an idiot, i dunno. But that's why I put it as a poll :)
Another situation can up recently where one of our team went AFK for a while because of lag, and let us know he was going to restart client because he needed to reset his router. After he asked us, we wished him luck and understood. We eventually lost, but didn't blame him. After the game, one of the enemies asked if we wanted to report the afk (who didn't make it back in time), and we said no, because it was network issues. The another enemy was like.. shit, I already reported him :o he apologized, but there isn't really anything to do at that point :/
I'm sure there are other scenarios as well.