TIL that the Shortest Reigning Head of State was Louis Antoine of France, who "Ruled" for 20 Minutes on 2 August 1830 as His Father Abdicated but Louis Antoine Refused To Sign the Abdication Document. This, however, is Disputed by some Historians who Claim it's "Too Short to Count".
https://en.wikipedia.org/wiki/Louis_Antoine,_Duke_of_Angoul%C3%AAme
