For me, it was a reoccurring nightmare that was more of a feeling than some story or vision. It’s really hard to explain, but it always followed the same pattern: I’m doing something (and not in a “doing” way, just some feeling of activity), then this activity somehow starts to speed up until it rushes at an unimaginable speed, and then suddenly comes the realization that this thing is endless and lasts forever, so the speed subsides leaving a lingering feeling of dread and despair.
I remember trying to describe it using this metaphor as a kid (again, nothing here is literal): I’m walking somewhere, first normally, and then speeding up, with walking morphing into an exhaustive and panicked run, until the realization that the destination is only getting further the faster you run hits, so I go back to walking, but feeling crushed at this realization.
I also remember describing or thinking about these dreams in other ways, like flipping through pages of some important book ever faster, until it dawns on me that the book is in fact infinite, for example. Or exerting more and more effort to think of some specific word until I realize that it doesn’t exist. Stuff like that.