If you have Rex's Curse and you get an Elder Sign token on the redraw and you opt to treat it as an automatic fail, does Rex's Curse get shuffled back into your deck?
I could see it going either way. One could argue that Rex's Curse caused you to fail because you would not have failed the skill test if you did not have the curse. One could also say that the curse did not cause you to fail because you could have chosen to succeed (assuming the elder sign would have been a success) and elected to convert the elder sign to an auto-fail.