Tyranny of the masses is the basic concept that sometimes the majority might be wrong.  The third reich and racism are two glaring examples taken from recent history.  It was a major issue for the founding fathers of the US and is still a real possibility.  The mass mentality can be particularly frightening because once people as a whole are headed in a particular direction, there is an incredible amount of inertia that can be very hard to redirect.  And yet the public can be fickle, paying attention to an issue at one point and then flip-flopping (or forgetting it) the next.