After unsuccessful attempt to occupy Serbia in 1914 Austro-Hungarian forces managed to settle there from autumn 1915 until the end of war. Military General Governorate for Serbia was formed on 1 January 1916 with General-governor Count Salis-Seewis residing in Belgrad. Serbia was divided into 13 administration parts - county commands (Kreiskommandos): first were Belgrad-Land, Valjevo, Šabac, Gornji Milanovac, Kragujevac, Semendria, Belgrad-Stadt, then Kruševac, Čačak and Užice and finally since 15 March 1916 Prijepolje, Novi Pazar and Mitrovica am Kosovo. Although Count Salis-Seewis was from old swiss noble family he was accused of being pro-slavic (no surprise - he was born in Croatia) and in 6 months was replaced by Adolf von Rhemen zu Barensfeld.
When things were settled in Serbia, Austro-Hungarian High Command decided to defeat Montenegro to rule them out of this war. Montenegrin armed forces which were battling alongside Serbians returned home and posed latent threat. During January 1916 Astro-Hungarian army conquered Cetinje, made Nicholas I to flee and signed armistice with remaining ministers of Montenegro government. On 1 March 1916 they established similar Military Governorate as for Serbia and appointed general Viktor Weber Elder von Webenau who served until July 1917, replaced by reserve General Count Clam-Martinic till the end of war. Administration of Montenegro was divided to 7 county commands (Kreiskommandos): Cetinje, Podgorica, Nikšić, Kolašin, Berane, Ipek (Peja), Plevlje.
Austro-Hungarian Empire attempted in both countries to devalue money to achieve exchange rate of 2:1 in favor of Austro-Hungarian Krone. The important part of it was validation of paper money by special handstamps by Kreiskommandos to restrict import of dinars to Serbia and perpers to Montenegro. Particularly in Serbia because Serbian officials took printing plates of banknotes when escaping from country. In Montenegro they were trying to get gold coins from public in exchange for paper money and finally printed in 1917 new paper money (with denomination already 2:1) and ordered to replace all existing ones in circulation.
From timeline point of view both validations ran in parallel therefore it is no surprise that erroneous validations happened. And for joy of collectors even survived all the years to be available to obtain it. Below you can see 1 perper from Montenegro with both Cetinje handstamp (Montenegrin Kreiskommando) as well as Belgrad handstamp (Serbian Kreiskommando). Second documented example I found in Katalog Novca Srbije i Crne Gore 1868-1918 by Vojislav Mihailović and Dragoslav Glogonjac from 1973 with series B44 133.
In case you are interested in this specific part of notaphily, you are invited to look at lots 353 - 386 with decent number of Montenegro handstamps (including arguably hard to obtain IPEK and quite rare 100 perpera first issue of 1914) and lots 528-538 with few Serbian handstamps.
I hope you enjoy some background information and I am already looking forward to preparing another post for you.