James at Nourishing Obscurity tagged me for this. Tricky question because I've done quite a lot of stupid and/or implausible things so I see no reason to limit myself to particular physical actions. Hence I think I'll go for a more philosophical or perhaps metaphysical set of things that I'll never do. Anyway here they are
Vote for a socialist of any description
Vote for a rise in taxes
Trust the government
Trust the media
Become a devout Moslem or Mormon
Or a devout believer in any other or pseudo religion (e.g. Ecology)