1 Eleven: So how do we open the Valves of Mithril? That bronze key right there on the step?
2 Dustin: Sure, go ahead. Take the obvious key and put it into the keyhole. What could possibly go wrong?
3 {beat; all stare at Dustin}
4 Dustin: What? I figure she’s got to learn from hard-earned experience at some stage!

This key is actually needed to complete the adventure. But not here!

Unless you consider dying by electrical shock to be an acceptable end to the adventure.