Raise your hand if this ever happened to you: you are having a charged e-mail exchange with someone in the company and then all of a sudden a third person pops up on the thread and chimes in.
What just happened? What the hell is this person doing here? It turns out that your pen-pal has been putting other people in the BCC!
To you it feels like a betrayal or snitching. Your first response is “why did you include ____ here without letting me know”, but you can’t really say that because you don’t want to make the new party feel like you’ve been hiding anything.
The worst thing is that the person doing the BCC is kind of embarrassed as well. They don’t put people in BCC because they want to show who they are copying.
Of course the BCC’ed person didn’t mean to cause any harm either. We are so busy nowadays that we just reply without diligently checking whether or not we are openly copied.
So I developed a great habit. Never to BCC people unless I am hiding everyone for e-mail address privacy. With a bunch of e-mail programs you can actually hide that evil field altogether. BCC is a sneaky and creepy way to include someone. Save everyone the hassle and just use Forward.