I can remember being accused of racism on this very site, funnily enough. There was a thread about affirmative action/positive discrimination/whatever you want to call it, and most of the discussion was perfectly reasonable, but this one guy just ranted at everyone who was opposed to it like they'd declared ardent admiration for the KKK. I hadn't actually been involved in the thread, but he pissed me off enough to make me call him out on it, and rather than even try to offer some kind of defence he just accused me of racism. Luckily the thread got locked before anyone said something mod-worthy.