I was running around in Limsa, when suddenly I met an invisible wall. Other players had already been running through there, so not a place I wasn't supposed to enter. I tried to run in a different direction, and ended up falling through the floor (or off the edge, I really don't know).
Next, I find myself on a lower level of Limsa, near a tunnel path. I tried running in there but kept meeting invisible walls here and there, and accidently stepped outside the edge and realized I could walk on water. New skill lol?

This happened around x:10 y:15, near the Armorers' Guild.



Dropped from up there: