My supervisor accidentally forwarded some private e-mail communication to me. What to do? My supervisor has wrote to another cooperating professor regarding our project and had put me in the loop (CC). But I believe he has made a mistake and had also forwarded some of their previous private important chats and communications. I am afraid if he was willing to share those information with me. Even if part of those are relevant, but one may consider this as sensitive information of these big guys. Is that my responsibility to write back to him and inform him about this? Most of times the sender will notice such a fault, at least after sending the letter, but what etiquette say about this? I cannot say "you have wrongly put me as a receiver," because he had told me "I will put you in the loop," but this was just for the last email, not for their previous communications.
This happened to me on two occasions, one of which would have been personally and professionally embarrassing to my advisor, or worse. 

I had a pretty good relationship with him, and it was very clear the content wasn't meant for me, so perhaps my situation was slightly different, but I said something to the effect of:

'Woah, looks like the horse left the barn on this one. Glad it was just to me!'

In doing this, I admit I understand the embarrassing/sensitive material, but make clear that I appreciate it was a mistake, and also give him piece of mind that I am a safe confidant that can keep something to himself.