I have on numerous occasions bumped into "the argument backwards from consequences" - whereby people argue that X cannot be true, because if it was true then this would result in consequences they would not like. I have seen it put forward at a Stephen Pinker talk a few years ago, when some socialists said that if people weren't totally malleable then we wouldn't be able to form a perfect society, therefore people's personalities _must_ be totally malleable.
That was the most glaringly obvious one until just now, where I read a comment elsewhere on LJ that said:
"
How are you going to argue this in any way that wouldn't cause society to collapse if you were right?"
As if something would cease to be right if the arguer didn't argue it, or the collapse of society was something the universe was designed to prevent. Not to mention that society clearly hasn't collapsed, so if the proponent of the argument is correct then clearly society can cope perfectly well despite this.
I have, in the past, been guilty of this myself, being sure that souls must exist in order for me to have free will. I feel embarassed that I ever "thought" in that manner.
Edit: Serendipitously I just found
this Charlie Brooker piece with the fantastic line
Science is like a good friend: sometimes it tells you things you don't want to hear.. Read the entire article. Then forward it to your friends.