Mind boggling horror recs?

Not sure what the profanity rules are in the group, so just to be safe, what are some great mind boggling horrors that had you thinking about them days later?

I recently watched Triangle and even though it had some elements I’d expected to happen, it was still a great watch and had me thinking a few days after about it.

I don’t know if it’s horror (probably more thriller) but I also watched The Thirteenth Floor and enjoyed its concept as well. Especially why they got amnesia.

But I’m looking for some recs for something that makes you stop and think about reality and how close the line might be between here and the ‘twilight zone’ - type places.

Bonus: Coherence is another good example.