Well..probably when all non-brick/tarmac surfaces (i.e. mud etc) suddenly lost all of their usual contact rules. Walking on them meant I sank, then popped up again, then sank. It got nauseating. It was at about the time I was floating away from a Giant Radscorpion that it started raining corpses. Raiders and Super Mutants fell from the sky in bits and pieces, some sticking a little way up and stretching to the ground, and then through it, so there were strange polygon arms about 40 feet high sticking out of the ground like macabre trees.
I also noticed I was being shot at through rocks from a Mutant the other side of a wall of boulders and a house. VATS could target him strangely enough.
I then bobbed under said rock wall, and I could look up to the inside of it, soon I dropped through the scenery only to find myself falling from miles above the map. I eventually landed somewhere near Oasis (which looks pretty good from above, I might add) and died from the fall.
Overall, one very trippy experience.