I have seen this bug very often since the recent updates (1017 being current).
On joining an instance the current instance master may appear to be sitting in a random chair in the world. The player will remain stuck in the chair for you until they sit in the chair and leave it.
This seems to happen to instances that have been open for a while, there does not seem to be any pattern to which chair the person is sitting in. This happens even if the player has never sat in a chair in the world. It only happens to the player the has been in the world the longest, so that is what I am calling Instance Master.