It's almost always just a geocoding/data format issue. It's machine readable but you can get quirks when an address is something like "1 1/2 Hacker Street" - is that "1.5 Hacker Street", "Unit 1 1/2 Hacker Street", "1 Hacker Street, Unit 1 of 2" etc. It might not be a problem if you are dealing with a small discrete area but when you are trying to merge multiple areas it can be a challenge.
Then you have situations where a street name is the same as the region/state (is the source data just missing attributes?), confusion about the direction prefix/suffix (1 W Hacker Street, 1 Hacker Street W - can vary within the same city on the same street), poorly named streets like "1 W Hacker Street East or "1 Hacker Street East W", or street name types that are not commonly used (1 Hacker Launch Pad etc).
It's even more fun when you start talking about countries that either use older addressing formats (I've seen addresses in Ireland that are just like (Old Blue Cottage, Some Town) or are not in english.
There are many more situations, these are just what immediately come to mind.
All addresses are pretty hard for computers to understand, so the people building the software to make them understand (geocoders) usually make them work for the most 'in-demand' address formats. Just like every other bit of software, finding the special cases and figuring out how to understand them is tricky.
Then you have situations where a street name is the same as the region/state (is the source data just missing attributes?), confusion about the direction prefix/suffix (1 W Hacker Street, 1 Hacker Street W - can vary within the same city on the same street), poorly named streets like "1 W Hacker Street East or "1 Hacker Street East W", or street name types that are not commonly used (1 Hacker Launch Pad etc).
It's even more fun when you start talking about countries that either use older addressing formats (I've seen addresses in Ireland that are just like (Old Blue Cottage, Some Town) or are not in english.
There are many more situations, these are just what immediately come to mind.