Most of the time when I say something "gives me nightmares," what I almost always mean is it is a source of intrusive thoughts during the daytime while I am wide awake.
For instance, when I say something like "I have recurring nightmares of Scott the Woz falling off my roof and me being cursed to transform into him all Santa Clause style," what I actually mean is that I made the mistake of making up that sentence up a couple years ago and it has been haunting my waking thoughts ever since. I do not mean to imply that forced Wozification has ever been the subject of any of my dreams.
Apologies to anyone I may have misled. I hope I can do better in the future.