Yeah, one time I told my 3 year old not to touch a wild mushroom. Mostly because I'm grossed out by it. But now she's got this crazy attitude towards mushrooms, the weirdest plants in the world that people eat but are dangerous. I have no idea how it's going to shake out. If I could do it again I would have let her touch the freaking mushroom.

Yeah, that's true, you definitely do have to be careful with that kind of thing. Kids really can take things to heart and believe them so strongly that it's difficult, even when they know you're the one who originally gave them the information, to moderate it. I usually try not to speak in terms of absolutes, but let them know what _can_ be dangerous, especially for kids who don't yet have enough experience to know which situations are dangerous and which aren't—so if they want to use/touch/whatever it, please ask an adult first. So it's not the thing itself that's inherently dangerous, but rather using it before you have enough experience. (Still do have to emphasize the potential danger though, lest they one day decide they do in fact have enough experience now.) This seems to work well for my kids, but of course ymmv.