Since the launch of Google’s Penguin update in April, sites that rely on black-hat link building including purchased links and link exchanges, have had their page ranking penalized.
The question has been, exactly how is Google deciding if you’re using black-hat techniques. The answer, based on past experience, is that they’re making an educated guess.
There have been plenty of horror stories circulating since Penguin was pushed live, ranging from sites penalized because an employee linked to their day job, to spammers suing over spam-links they inserted themselves. Most of these are just scare stories, but genuine issues have arisen.