To be irrational is to be human. Well, technically, rationality is human as well. It is all a matter of thinking. Thought processes range from imagination to logical analysis--but all are human activities. So what is the point of all this?
Well, to illustrate, a friend of mine who tends toward very creative thought processes once explained to me why he thought that a drinking binge helped him think more clearly afterward. "The weak brain cells are the ones that are killed off by the excess alcohol..." An excellent example of rationalization.
What makes validity, though? Rational thought can definitely be flawed. I once took a Metaphysics class. In one class session, the professor used logic to prove both the existence, and the non-existence, of God. Another source of weak validity lies in faulty scientific experiments. There are many different ways that an experiment can be flawed in design or conduction. More information about experimental error can be found at http://www2.volstate.edu/tfarris/PHYS2110-2120/experimental_error.htm. The point is that validity is not inherent in either logic or science. Another way that we can be fooled by rationality is found in statistics. There are actually a variety of ways to interpret the results of statistical analysis, and even the way that the analysis is done affects the outcome. It is very important to remember--don't believe everything you read, hear, or even see. Hey, I forgot, there are also visual illusions...the list goes on beyond this I am sure. Randomosity lives on!