In The Eleventh Hour, the newly regenerated Doctor is trying to solve several mysteries at once, and gets stuck on the fact that there's a duck pond in Leadworth with no ducks. Nothing comes of it at the time. Later on, he's attempting to reason out the Cracks in Time, and mentions the duck pond as a clue in the mystery.
For some reason, some people are still stuck on the duck pond. The answer is the cracks in time ate the ducks, which foreshadows the cracks in time having eaten Amy's family, which is itself a Pond without any Ponds in it.