Watch out what you say on social networking sites. You never know when a comment could come back to haunt you. Public memory may or may not be short, but the evidence lives on.

"I apologize for my past improper words. I love my motherland and I have never harmed its national unity," Qiu Chen, a well-known public debater, pleaded on domestic social networking sites on Monday evening. "I will close my social networking accounts and stop appearing on public programs."

Accused of supporting Hong Kong separatists, Qiu, at first, dismissed it as "rumor". However, one micro-blogger after another posted snapshots of his past online comments, including a photograph showing him with separatists. Finally, Qiu gave up.

Qiu's comments cannot be protected under "freedom of speech" as some argue. In China, as anywhere else, freedom of speech is protected, but it has its limits. Separatism cannot be pardoned because of the freedom of speech.

Even in the West, those supporting controversial figures have invited criminal charges. And the internet remembers everything; it is impossible to erase what's been said online. Qiu himself might have forgotten what he posted or commented online in the past, but it still exists, and can be dug up and reposted.

That is why public figures should be extra cautious and avoid commenting irresponsibly about things they don't know much about. A loose comment anywhere can always come back to haunt someone later. Social media allows everybody to express their opinion on matters of public interest but only self-regulatory behavior conforming to the law will prevent the online public forum from becoming a lawless jungle.