In many online discussions and debates, I read frequent examples of ad hominem or also known as argumentum ad hominem. This type of fallacy is meant to attack the character of a person instead of pointing out errors in their argument or premise.
Often, people will use this type of fallacy when it comes to linking to sources. Instead of dealing with the points made in an article, lazy people will often resort to trying to attack the publication or the author of an article.
While character of a person is important, it does not mean that an argument they have made must be wrong if they have made mistakes in the past, or have been wrong, previously. It may even be factual that a person may have a bad character (subjectively or objectively; for example, they may be a known thief but that does not necessarily mean an argument that they make about whether a law is just or not, must be incorrect due to their character.
Often it can be very frustrating to deal with people who resort to ad hominem fallacies in their discussions. It means they are too lazy to think about what is actually and precisely being communicated.
People who resort to these arguments ought to be called out on them, even if you happen to agree with their side of the debate.