At the "Shadows in the desert" chapter in Act 2, the Enchantress has trouble following at the first section of the Desert, where you clear out the 2 Cellars, and then cross the bridge. At the bridge, she stops progression to tell you about herself, but the quest is not available for you to click.

Its fine for me, because I can just jump past that part by advancing the chapers at the character selection screen,, but for anyone's first play through, they are stuck. Also, anyone trying to stack Nephalim Valor is SOL.