Here is a particular example from my fanfiction, before I caught it during proofreading:
I wrote: "How did you sleep Chomper? Oh, that's great. I just had a sleep story where you
date me."
I meant: "How did you sleep Chomper? Oh, that's great. I just had a sleep story where you
ate me."
Although I suppose that either one would constitute a nightmare from Littlefoot's perspective, it is only the second one that I had in mind. The first option would most certainly not be featured in any fanfiction that I would write.

There is more than enough disturbing pairings in fanfictions without me adding to the list.