It could be literally anything. The lawsuit by RealNetworks against Streambox over the Streambox VCR software established as legal precedent that one single bit, undocumented, is adequate to qualify as a "copyright control mechanism" and that if you ignore that bit, whose purpose you need not even be capable of knowing, you have circumvented it and broken federal law. (There is some sweet irony in the fact that the DMCA, which the Realnetworks v Streambox suit first established the legitimacy of in the courts (due solely to Streambox running out of money - they won every case until they could not afford to respond to an appeal, handing Realnetworks a default judgement) was later used against Realnetworks when they enabled their users to backup DVD movies.)