I've been reading How To Outnegotiate Anyone and read a paragraph in particular that made me laugh:
The "off the record" comment In negotiations, whether you are making an offer or not, there is no such thing as an "off the record" comment. If the other side hears it, you might as well write it in your own blood or, perhaps more appropriately, have it engraved in your tombstone. As with the informal proposal, the simple rule here is to never say anything that you haven't thought about carefully. In other words, don't open your mouth before you engage your brain. This is a common problem with American negotiators. We love to talk talk. In fact, we can't shut up. As such, we tend to easily overstate or understate our case, or otherwise miscommunicate our position to the other side.
So true - I've seen this first hand many times.