Medievalists.net has an interesting little article about a number of international boundary curiosities that originate in the medieval era. It can be seen at Strange Borders with Medieval Origins
There is more about the delightfully intertwined towns of Baale Nassau and Baale Hertog on the Dutch-Belgian frontier at Europe's strange border anomaly
The history of Ceuta is also set out in a recent article at The curious slice of Spain in Africa
The Franco-Spanish condominium of Pheasant Island, which changes its national administrative allegiance twice a year is not included, presumably because its status was codified in the mid-seventeenth century. There are online articles about it at Pheasant Island and Europe's island that swaps nationalities
At a local level in this country the nineteenth century had an unfortunately strong tendency for to tidy such oddities in county boundaries up, and those which survived tended to be casualties of the dreadful 1972 Local Government Act. One such was Dudley - a detached part of Worcestershire surrounded by Staffordshire, with Dudley Castle at its centre as a detached part of Staffordshire. Similarly York Castle was a detached portion of the North Riding in the City and County of the City of York, and similarly the Nottinghamshire County Hall in the middle of Nottingham, and was constituted a civil parish in its own right.