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alternatehistory.com
What is the craziest borders you have in your setting? How did it come about? Use as crazy an alternate history as you please.
One of the borders I have in a timeline would be a panhandle that is set 1 meter off the ground. If you try walking towards it you'll have your upper body illegally cross the border and will promptly be tasered by the border guards, returning you back over the border as your body hits the ground.
Due to timeline shenanigans the crawl space beneath a building belonged to one nation whilst the first floor belonged to another. The building was culturally important to one nation but the other nation wanted the burial site located underneath. The building is now gone and a poorly worded treaty maintains the border. The locals consider it sport to trick people into walking across the region.