Although no catastrophic events have been observed from geological evidence of past flips. A flip in the magnetic field could cause mayhem with the world's electricity grids. On top of this, since the Earth's magnetic field protects us from cosmic rays and high energy particles from the sun, there could be an increase in cancers from genetic mutations caused by the weakening of the earths magnetic field. However it should be pointed out there is no threat of this happening in the next 200 years so we will have plenty of time to prepare for when it does happen.
Source (University of California Berkley)