In case you have never heard of a logical fallacy, they are quite simple to understand. As the dictionary defines it, a logical fallacy is "a misconception resulting from incorrect reasoning."
The Left loves to roll them out all the time, as I am certain you are aware. Today, however, with President Obama declaring executive privileges on damning documents concerning Fast and Furious, liberals have relied very heavily on one specific fallacy: tu quoque.
Tu quoque, or "you too," is very simple to understand as well. It's pretty much being a hypocrite; it is when someone does something wrong, then says that no one can correct them because they did it too. So, for example, if a former smoker tells someone to quit smoking because it is unhealthy, the other person will say, "You used to smoke, therefore you have no authority and smoking is healthy."
When it comes to Fast and Furious and executive privileges, the Left attempts to defend the indefensible by arguing tu quoque as much as humanly possible. One extremely common fallacy is displayed below, taken from the Huffington Post:
And guess what: liberals sticking by the President for the sole reason that "Bush did it" are the lowest kinds of mental midgets. Kind of like this man: