Here's a scenario. You are sending emails out and CCing someone. That someone replies back and the person you had on CC replies back to them, because they apparently know this person, about how they should totally do lunch sometime soon! Teehee, great. I'm still in the email thread. Then one of these two parties is all "yeah let's do lunch but let's talk about it offline" or "in another email". Okay you just made me feel awkward. Obviously I am not invited and there are just three people in the conversation, let's just ignore that third random person who is unwanted.
This doesn't necessarily fill me with rage as the rest of the entries in this blog but seriously WTF.