For example, an explorer discovers the Coal Chute, and the Coal Chute leads into the Chasm. So which side of the Chasm does the explorer land on? Do they then have to cross the Chasm to get back to the rest of the house?
If it is the Vault you always appear on the outside, otherwise it is your choice as to which half of a barrier room you appear on.
From the official errata & FAQ: