Broken Base: Despite its antiquity, the rules of Go are mostly the same everywhere, but there are technical differences between the rules used in China and those in Japan and Korea. The differences mostly concern the way the score is calculated, but also how a few very rare situations are handled.
Scrub: Some players feel entitled to assume that their way is the only legitimate way to play, and anything else is cheating. The particularly applies to the choice of Japanese vs. Chinese rules and scoring.
That One Rule: There's a major rules dispute over how to handle endgame ko fights. Japanese rules forbid ko in the endgame, which makes certain shapes dead in certain situations where they might survive. Chinese and American rules have no such stipulation; if life and death is disputed, the fight is played out instead of virtualized.