If you spend enough time putting stuff on the internet, you will eventually:
1. Say something about someone that just ain’t true;
2. Get a few details wrong about a person, or a situation involving that person; or
3. State an opinion about someone, which that person finds objectionable.

Any of these could result in the threat of a defamation suit, but only #1 has any real chance of going badly for you. Regardless, you have to respond if someone doesn’t like something you wrote and subsequently accuses you of libel. (I know of what I speak. Just trust me.) You even have to respond if someone accuses you of slander because of something you wrote, and pointing at that person while laughing is not a sufficient response.

Ken White of Popehat fame has compiled a helpful list of steps to take if you receive any sort of notice, even an incoherent or delusional one, accusing you of any sort of defamation. It is not legal advice, because legal advice is. not. free, but it’s very helpful nonetheless.