One of the major character background elements for Amy Pond's during her time as companion was the fact that she grew up with a crack in the universe in her bedroom wall. This forms the key plot element of the first series with Matt Smith. Eventually, this crack in her wall closes in "The Big Bang" as a result of
The Doctor sacrificing himself and allowing himself to be absorbed into the crack
at which point we see it seal itself off.
However, about 10 minutes into "The Eleventh Hour", The Doctor closes the crack in Amy's wall by forcing it open, then allowing it to shut. He even explicitly points out "See, I told you it would close, good as new." as we see it appear to seal shut in the same manner as "The Big Bang".
The scene in "The Eleventh Hour" happens just before The Doctor leaves Amelia Pond waiting in her garden for him to return; the scene in "The Big Bang" happens just after Amelia falls asleep in the garden, waiting for The Doctor.
So what caused the crack to re-open in that short interval?