What if you use BSD-licensed code, only it later turns out that the person who added the license wasn't the copyright holder? What if you ask someone if you can

What if you use BSD-licensed code, only it later turns out that the person who added the license wasn't the copyright holder? What if you ask someone if you can have something, and he says yes, but later it turns out it wasn't his to give away? The license is irrelevant; you are worried about people claiming something is under X terms, when it's not. The answer to this is: do your legal due diligence like everyone else. (The only wrinkle is that in some jurisdictions it's not legally possible to put something into the public domain. This does not seem to bother most people.)