I'm not a huge mushroom person...but basically the dangers would be.. You got ripped off, and got some bad mushrooms and could have a very bad reaction. Not a bad trip, but have seizers, possibly(but almost never) die, or something.
You might have a "bad trip", which would basically be a nightmarish experience only you're awake. So fear, paranoia, crap like that.
Nausea isn't too uncommon, some people say that throwing up will intensify your high. I'm not too sure about that, since I've never thrown up from them, but I have felt like I was going to.