James Potter kicked Lord Voldemort in the nuts.
This occurred in the early morning of October 31, 1981. Acting on a prophecy concerning Potter's son, and having secured the necessary means of entry from a certain backstabbing rodent, the former Tom Riddle entered the premises in explosive fashion ready to deal with any magical resistance in a fatal manner. Instead he found himself confronted by James Potter holding nothing more than a half-eaten crumpet and a cup of tea. Seeing his opponent with no wand or other magical paraphernalia to speak of, Voldemort naturally assumed that he had the upper hand in their confrontation.
He neglected to consider that he was faced with the most egregious juvenile offender to have ever graced the halls of Hogwarts School of Witchcraft and Wizardry. Wasting no time on pleasantries, Potter simply hauled off and delivered a swift boot to the scrotum that left the Dark Lord curled up in the fetal position.
As it so happens, Voldemort's wand took this altercation to mean Potter had triumphed over its master in an official magical duel, and as a result, switched its allegiance to James in accordance with his victory. Neither of the two men had any knowledge this had occurred, so when Voldemort then attempted to use the Avada Kedavra spell against his attacker, the resulting backfire blew him into so many pieces the autopsy conclusion was 'random act of God' and 'you're so bloody smart, you try putting him back together!'
With new wand in hand, James Potter subsequently assumed control of the Death Eaters. He then proceeded to engage in a campaign of tyranny lasting two days, which included commanding half the Death Eaters to 'go Alohomora themselves' followed by a complete sack of Gringotts Bank 'just for the bloody hell of it'. This dark reign ended upon receiving a Howler from his wife which told him to 'knock off this silly business and come straight home before I give you a kick in the jimmies!'
The report that before leaving Potter fed a large rat to what many insisted was suspiciously similar to Minerva McGonagall in Animagus form cannot be confirmed.