I'm having a similar problem when going from act2 to act3.
When I go from act2 to act3 is suddenly have a teleporter stone in mine inventory. I didn't find any teleporter stones before, and I'm sure I don't have a teleporter stone right before I talk to the imp that shows you the way to the raanaar temple, and where you then go to act3. Maybe when I go to act4 I have 2 teleporter stones.
This doesn't bother me or so, since it doesn't make have any difference to me wether I have one teleporter stone or none, I just found it weird.
