First time was early on, using the Karlach trick to stop close-out of things like the Grove story. This trick works, and you can then go back and the Grove is still vibrant with tieflings, you can then get the goblin leaders at your leisure. Heck, Auntie Ethyl is still selling stuff in her alcove, hasn't even left for the Teahouse, yet.
I've done this several times, it works just fine.
But...I may have gone back into the mountain pass to pick up the periapt, as it's right there. It's possible this new re-entry did it. It did warn me of finishing up stuff before letting me through, but I assumed you could go back and forth at will. I may have been wrong.
