What happens if I get an "I am error" room in the Backasswards challenge?
It seems that this is actually an issue that has not been solved yet. Looking at this Reddit comment:
IMPORTANT ISSUE: I Am Error rooms break Backasswards
Finding an I Am Error room during the Backasswards challenge leaves you unable to continue, as the only way to exit is to take the trapdoor down. Problem is, Backasswards forcibly closes all downward trapdoors, so you're just stuck.
So you would remain trapped in this room, and wouldn't be able to continue your run.