Someone downvoted this, so I'll provide this quote from the UK FCO:
> Taxi drivers from the Republika Srpska might be unwilling to drive to a destination in the Federation, and vice versa.
https://www.gov.uk/foreign-travel-advice/bosnia-and-herzegov...
Or Wikipedia
> police conduct around the Inter-Entity Boundary Line separating the two entities of Bosnia and Herzegovina, the Federation of Bosnia and Herzegovina and Republika Srpska, had been the "greatest obstacle to freedom of movement", including intimidation and arbitrary fines.
https://en.wikipedia.org/wiki/Vehicle_registration_plates_of...
There are plenty of anecdotes similar to mine on TripAdvisor etc.