An instance where someone unknowledgeable, incompetent, or otherwise unqualified is giving direction or advice to someone in a similar state of inability.

One of my favorite passages from the Bible's new testament, Matthew 15:14, reads "If a blind person leads a blind person, both will fall into a pit." Jesus said this in reference to the Pharisees who were insisting that Jesus was committing a sin by not washing his hands before eating, breaking a religious rule which, although unhygienic, is hardly a deadly sin.

The expression refers to those who pretend to be the keepers of the absolute truth -- ususally they lead their followers to their downfall, although it's not usually as black-and-white as that.