I don't remember many details, but it was definitely a Good Omens/Harry Potter crossover. Apparently, Crowley and Aziraphale were teachers at Hogwarts, and the scene that I remember the most clearly is that Harry and Ron couldn't do something - I want to say enter a classroom, and it was like they walked up the staircase and opened the door and were prevented from going through it, either by a physical barrier or by being unable to move themselves forward. It turned out that they were being punished by Crowley for something they'd done earlier; they couldn't do something they wanted to do because they'd broken the rules earlier.
The thing that made this dream weird was that as soon as that happened, I thought to myself, "No, that's not right. That's out of character for Crowley. He wouldn't punish them for doing something bad." (My husband points out that he would, if it was a favor for Aziraphale, but that's beside the point.) So, I rewound the dream and started that scene over again, though I don't remember how it turned out.
That was the weird bit, controlling the direction of the dream. I know it's possible, but I d never had it happen happen before. I have had dreams rewind themselves and play out the same scene over and over (sometimes with changes), but not at my command.
Of course, now that I've told my husband about it, he's pestering me to write a GO/HP crossover. Sigh.