747 Junkyard argument
The tornado argument states that evolution is like a tornado moving through a junkyard and assembling a fully-functional 747. This analogy plays off the percieved "randomness" of evolution on the part of the creationists.
The problem with the analogy is that evolution doesn't work in massive jumps like a tornado would. Evolution has no goal; instead, it works with what already exists and modifies it to accomplish new things.
Intelligent Design advocates sometimes present calculations showing the impossibly-low odds of a given protein spontaneously self-assembling from a batch of amino acids. William Dembski used it in his paper, Specification: The Pattern That Signifies Intelligence. Such calculations are irrelevant because they ignore important features of proposed evolutionary mechanisms -- the ones that get around such seeming impossibilities, in fact.
The tornado argument works on the common fallacy of confusing "natural" and "random". It is a false analogy for evolution by natural selection because it lacks two of the crucial elements: reproduction and selection.