One important aspect of any field of science is the ability to correctly state your position. To be considered scientific, a position must be based in reason and logic. A logical fallacy occurs when this requirement is not met. Since a clear, concise, and correct argument is important in science I am going to be highlighting a few possible logical fallacies. First up is a very common one: Ad hominem.

An argument ad hominem is an argument against a person instead of against that person's claim. There are three major types of ad hominem arguments:

Abusive ad hominem (Instead of addressing the real claim, you attack the person making the claim) 
Example: "I can't trust a word you say. You're fat and lazy"
This is a logical fallacy because being fat and lazy has nothing to do with someone's honesty.

Circumstantial ad hominem (Assuming that a claim is irrelevant because a someone is more likely to make that claim)
Example: "The dealer says my brakes need repaired, but he just said that to get more money from me"
It may be true that a slimy dealer may convince a customer to pay for unnecessary repairs. This does not mean that any repair suggested is unnecessary.

Tu quoque (You too!)
Example: "You can't tell me cigarettes cause lung cancer, you smoke two packs a day!"
A person's actions may be contrary to their claim. This does not make the claim invalid.

It is important to note that not every ad hominem argument is invalid. In some cases a person's character, intelligence, or personal beliefs are relevant to the claim. In these cases it is not considered a logical fallacy to use ad hominem arguments.
Example: "Sarah Palin seems to have no knowledge of foreign policy. She would not make a good president¹." 
Also, simply attacking your opponents character, intelligence or personal beliefs is not an ad hominem logical fallacy. The attacking must be used to attempt to refute a claim your opponent has made. That being said, you can call me fat and lazy all you want - just don't say that's why you can't trust me².

Notes
[1] I'm wondering who will argue with me because I said this.
[2] Ok...I guess if you say I'm lazy and that's why you can't trust me to do a specific task that wouldn't be a logical fallacy.

BONUS WEBCOMIC!! Check out Surviving the World by Dante Shepherd.