Belgium (different languages, laws, educational systems, public broadcasters per language region, taxation, etc.)
UK (different laws in Scotland, different laws in Northern Ireland, education policy, etc.)
Spain (autonomous regions with their own languages, seperate civil law in Catalunya, tax collection in the Basque country, etc.)
Canada (IRC Quebec has a Napoleonic inspired civil law system, whereas the rest of Canada uses common law similar to that found in the US and UK. TLDR one legal system uses precedent, the other doesn't. )
China (the unofficial city tier system, Xinjiang, Tibet, etc.)
Russia (autonomous regions in the far east, Kadyrov/Chechnya: strict alcohol prohibition and possibly years in jail, etc.)
India (IRC autonomous administrative divisions can make their own laws, tribe/caste based laws/tribunals, Jammu and Kashmir which until quite recently had its own seperate consitution and for example Indians from other regions weren't allowed to buy land or property there.)
The problem is that as a foreigner, you're usually ignorant about all these things. Whether it's a Brit who thinks all Americans are Yankees, an American who thinks all Brits are English, a Scotsman who thinks Spanish and Castellano are synonymous, or a Spaniard who goes to Belgium expecting to speak French everywhere.