There are invisible walls in game, at some quite important places.
The first one I've encountered is in the Culling of Stratholme instance, we can't go out of the kitchen in the inn where we are at the very beginning of the instance.
There's also some kind of invisible wall in the long staircase before the entrance of the Scarlet Monastery, but this one we can go through by moving a lot.
I'm quite sure these are not the only ones, but I can only remember these right now, sorry. If I see another, I'll post another ticket.