for me, the first thing about this question that has to be dealt with is:
can ideas themselves be dangerous?
but if one assumes that ideas inevitably lead to their implementation in action/behavior (i do not, but i know many who do), then i would think there are many, many, many. starting with the idea that humans universally are not capable of dealing with life (including conflict) on their own, and need leaders and codified authority. beyond that, i find most -isms to be dangerous to anarchic relations. possibly even anarch-ism. but then, i have been called (probably not entirely inaccurately) ideologically opposed to ideology.
edit: i would add that (for me) everything is contextual, and so some idea may be dangerous to someone at some particular time or circumstance; but not necessary for everyone or all the time.