Dreams Idea
Could things bleed through peoples' dreams? We know that only their Outie versions truly sleep; when we dream at night, part of the content comes from our brain reviewing the events of the day, and what's on our minds. Whether or not they know it, I'll bet that Outies have dreams related to what they do in the daytime at Lumon (especially particularly traumatic or memorable events).
We actually already know that something like this occurs- when Irving is falling asleep at his desk, he sees the black paint (without really knowing what he's looking at). Maybe this is how his Outie 'remembered' the bad elevator hallway; it distressed him so much (and/or happened so frequently in his Innie work life) that it popped up in his dreams. I was thinking of this because I have Nightmare Disorder, and so do many other folks- and it's particulary tied to traumatic events and PTSD for most people. We know that everyone in MDR (if not everyone severed in general) has experienced a rough time in life, or is otherwise in some kind of unpleasant situation; it stands to reason some of them would experience dreams, including bad ones. That's it, that's my shambolic rube idea!