Dreams flowing into each other over nights.
Have you ever experienced recalling your dreams and think "Wait...wasn't that the night before?" This usually happens to me when I'm recalling one dream, and I go "Oh, yeah, I had another dream earlier on" and then I think "Wait a second...wasn't that a few nights ago?"
I suppose you'd only experience this if you did not keep a dream journal at the time (since you can just look back at it). I have a pretty good dream recall without a journal, so I can remember my dreams for days with little loss. Many times I've been recalling a dream after I wake up and have to actually sit there and think whether that was last night, or the night before.