We probably all indulge in them from time to time. Sometimes intentionally, sometimes unawares. They are part and parcel of life on a forum such as this. There are always a few habitual offenders.
There are a lot of good sites on the web that have begun to compile them for quick reference. One of my favorite is at the Nizkor Project.
Here is one that I have seen quite a bit of lately.
Poisoning the Well
This sort of "reasoning" involves trying to discredit what a person might later claim by presenting unfavorable information (be it true or false) about the person. This "argument" has the following form:
1. Unfavorable information (be it true or false) about person A is presented.
2. Therefore any claims person A makes will be false.
This sort of "reasoning" is obviously fallacious. The person making such an attack is hoping that the unfavorable information will bias listeners against the person in question and hence that they will reject any claims he might make. However, merely presenting unfavorable information about a person (even if it is true) hardly counts as evidence against the claims he/she might make. This is especially clear when Poisoning the Well is looked at as a form of ad Homimem in which the attack is made prior to the person even making the claim or claims. The following example clearly shows that this sort of "reasoning" is quite poor.
Bill: "Boy, that professor is a real jerk. I think he is some sort of eurocentric fascist."
Prof. Jones: "...and so we see that there was never any 'Golden Age of Matriarchy' in 1895 in America."
Bill: "See what I mean?"
Jill: "Yeah. There must have been a Golden Age of Matriarchy, since that jerk said there wasn't."
Examples of Poisoning the Well
1. "Don't listen to him, he's a scoundrel."
2. "Before turning the floor over to my opponent, I ask you to remember that those who oppose my plans do not have the best wishes of the university at heart."
3. You are told, prior to meeting him, that your friend's boyfriend is a decadent wastrel. When you meet him, everything you hear him say is tainted.