Perhaps a little far afield from the original topic, but I'm always more cautious/concerned about potentially getting the leans than flying into zero visibility (clouds or just blackness). At least the latter ought to be unambiguous, or at least should be. I fly in very mixed terrain, from bright urban to dark rural, from flatland to mountains, and over small bodies of water. I always make it a point to correlate what I'm seeing with known terrain features. If it seems "slanted" should it seem "slanted"? That sort of thing. Heck, even in the daytime this issue can come into play, if it's very hazy and an opposite shoreline seems "slanted". I've never had the leans, but how it might happen is definitely obvious.
Getting sucked into "black holes" is also a big one. I was once with another pilot, who was PIC, going into a pretty big airport with a perfectly serviceable PAPI. There was this large, completely black area for a mile or so directly in front of the runway. He kept going below the optical glide slope, entirely fixated on that patch of blackness, even though the PAPI was perfect on that severe clear evening. I could not understand it, but it happens. Experienced it again with someone else PIC at an unimproved, but lighted, LZ. Again, could not understand it. Fixed the problem both times, of course, else I might not be here!
The only time I've personally been spatially disoriented was one night, transitioning through Class C, with a Southwest 737 on approach to that airport. Conditions were severe clear. No problem flying straight and level, it was brilliantly lit urbanization below. But I could swear that 737 was on a collision course, the lights were so bright, and the relative motion was just so, but of course we were well separated by ATC, and he passed 2000 feet above me. That sure was weird! It's never happened again, since, but I remember that feeling like it was yesterday.