Plot Wormholes

Too often I have seen books where this happens to a character, the barrier to their achieving a goal has an easy solution the author doesn't address, it can make me stop reading.

That said, sometimes it doesn't even cross my mind. Like Frodo jumping on an eagle and flying to Mordor never crossed my mind till I saw a joke about it years later. So that means I was totally buying the limits Tolkien had put on the world. What steps do you take to protect your plot from developing a wormhole that would be an easy short cut for your character?