Plot devices that are a little too useful

If you're a child, or a smuggler, and you live in, or have something to do with a large mansion in the English countryside, then you wouldn't think it entirely out of the way to discover, say, a conveniently placed secret passageway behind the fireplace thus allowing you to carry out your heroic or nefarious plans.

But even then, you might think that this sort of thing is getting a bit ridiculous:

They heard him fumbling on the wall, and tapped again, to show him where they were. Suddenly, there came a click, and bright cold light and icy air rushed into their hiding place.
'I always wondered why that great knob was there on the wall,' James said. 'Well laws, miss! To think of your really finding the secret passage!'

Joan Aiken, The Wolves of Willoughby Chase