Ok, I'm gonna spam the forum one more time tonight with this link: The Forgetting Pill Erases Painful Memories Forever | Wired Magazine | Wired.com

An article about the neuroscience of memory - how it's created, stored and recalled, and the fact that it's really isn't as rock solid as we think it is - in fact each time we recall a memory we actually have to rebuild it, and it can be changed slightly (or more significantly) in the process. So this explains why dreams can alter memories pretty clearly I think.