When is it best to confront people or best to leave it alone?

The most recent situation just happened about 2 mins ago. I work with my bf--and we are supposed to be totally honest with each other (aren't all bf's?) about work, and personal. If we don't want to answer anything, we don't lie, we just say I can't talk about it. Well, I've been left alone here in the office to take care of some work, and quite unexpectedly a letter popped up---totally opposite of something important that this friend had told me. The kicker is---it was written 3 years ago. SO--do I say something now that I saw this, didn't mean to, but am bothered that I was lied to back then? Or just let it go? In the past this happened--my mother had asked me to help her clean out her computer--and when I did---found several saved im conversations between her and my DH about me, both against me. DH and I were having some marriage problems at the time. I let it go then and never mentioned that I had seen it. Do I just let this go too?