My biggest pet peeve is the common misconception that scientific theorems (or paradigms, even) are like a tower of cards: you find evidence against one part, therefore the whole theory collapses. This is especially evident in the climate change debate. One piece of evidence does not destroy an entire paradigm.
I'm guessing that's not what you want by this though, judging by your previous posts.
Maybe something like the idea that fats are terribad. I don't think a lot of people really understand the difference between transfats, saturated fats, etc. and how important fats can be, biologically.
I don't really understand your opposition to the evolution one though? Maybe just say from the outset "If you don't think evolution happens, GTFO" at the beginning

'cause it'd be interesting to talk about evolution not necessarily about getting more complex, or "better", always, but just being enough to survive in their current environment.
Oh here's another one that another thread reminded me of. The fact that apoptosis is bad and scary. Sure sounds it. But cell death is awesome and so important.
Edited by Tanuki, 04 July 2011 - 06:00 PM.