Alright, so I had a very nice, well balanced mix of instruments just waiting for the vocals to be added. According to the main output, nothing goes beyond -3 db, I´m trying to keep a decent head room.  
 
I processed the vocals as I always do. The meter on them shows that I´m well below minus -6 db.  However, the main output peaks all the way to 0 db the moment I add it to the rest of the instruments. I know I'm adding additional data to the sound pool with the vocals, but it seems too much of an increase over the general mix.
 
I've checked for any hidden  saturation or twisted knobs, but nothing seems to be out of place. Not even in the main output bus. Any suggestions?

Yeh, it's kind of counter-intuitive...you initially expect that if no individual track exceeds -NdB then the mix shouldn't exceed that, either. But there's a reason we call it "summing". At any given moment, the numeric value on the master bus is the sum of every sample in every track. Sometimes they cancel each other, sometimes they add, but it's a fair presumption that at some point there'll always be peaks that exceed any single track's maximum value.