We had lots of trouble with WordPress bot-logins from Russia and Ukraine, so I decided to block those ip-ranges.
Turns out one such block was also partly being used by customers in my own country. I received some vague mails about some things not working correctly. So I removed that ip-block, and sent back some vague replies that it was a firewall that was too strict.
There might be other blocks listed as from Russia and Ukraine, that are actually being used elsewhere.
Anyway, with the advent of ipv6, the whole idea of ip-blocks might change.