Geomagnetic reversal
The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago; and may have happened very quickly, within a human lifetime.  A brief complete reversal, known as the Laschamp event, occurred only 41,000 years ago during the last glacial period.  That reversal lasted only about 440 years with the actual change of polarity lasting around 250 years.  During this change the strength of the magnetic field weakened to 5% of its present strength.  Brief disruptions that do not result in reversal are called geomagnetic excursions.