The Anti-Vax movement.
It's one of those crazy, unfounded, ideas that makes me very angry, even more than normal, because I was taken in by it for a while. I heard it in passing, and the idea of "Oh yeah, we really do pump our kids full of so much stuff so early" stuck. Over time it evolved to "Huh, maybe there's some credibility to this, so many injections so young can't be healthy for them. I don't have kids yet, but I'll look carefully into which vaccines I'll allow." It might have progressed further, but Mike brought it up at one point, I voiced my opinion, and he (nicely) ridiculed me out of it. Since then I've done my research, and I'm much more hesitant to accept an idea just because it sounds plausible.
Here's Penn and Teller's take on it (NSFW language):
The Anti-Vax movement brings up a difficult question for which I can see two answers:
1. We combat the crazy idea early, with the outcome of debunking it or giving it credibility where it would otherwise have none.
2. We ignore the crazy idea, and allow it to fall off the radar unnoticed or, in the case of the Anti-Vax movement, gain a large foothold.
Which do you think works better? Is there a third option that I've missed?
Never stop questioning,