When a dream gives you an odd feeling but you can't remember what the dream was even about.
Man, I've had dreams that have left me messed up all day, like really depressed or angry and sometimes angry at a specific person, but I can't even remember the dream properly.
It's really weird. You'd think that knowing whatever the event was that caused that mood didn't even happen would mean you could easily snap out of it, especially if you can't even remember what that imagined event was, but nope.