Two stupid ideas
Whenever we complain about stupidity, what we usually mean is someone or something is willfully ignoring or is mindlessly oblivious to something obvious. Whether something is obvious or not depends somewhat on your general understanding of the world. So if we declare that an idea is stupid, what we mean is either 1) It's based on an understanding which we reject, or 2) there's something immediately untenable about it, so much so that anyone who claims to hold it is either lying, refusing to think, or has something seriously wrong with them. It follows then, I think, that people who spend a lot of time raging about stupidity are people who find it difficult to understand other people and other ideas. Complaints about stupidity are often themselves quite stupid. Stupidity is still a real phenomenon though, and sometimes it deserves comment. Here, for example, are the two great but stupid ideas of our time:

1) Freedom, taken as the ultimate goal or as an end in itself. Freedom is only worth having insofar as it makes us free to do what is worth doing, and what we ought to do. As an ultimate goal without any regard to what is worth doing - dismissed in a post-modern fog of "what's true and good for you isn't true and good for me" - it becomes the freedom to be free so long as it doesn't infringe on the inviolable freedom of other free and equal supermen.

2) Equality, taken in the empirical sense. People are different and those differences matter. Those differences cannot be papered over, nor can they be legislated out of existence, or erased under the right conditions. At the level of particulars, there is no good reason at all to believe that the distribution of intelligence, inclinations, or aptitudes is uniform between the sexes or between racial groups, and plenty of good reasons to believe the contrary. Nonetheless, if you make brief and entirely defensible comments about this simple truth it's enough to get yourself dismissed from your job and targeted for a witch hunt.

The two ideas that so dominate our contemporary politics, morality, and cultural discourse - so much so that to question them is to place oneself outside the pale - are stupid. That is, they are either willfully ignoring or are mindlessly oblivious to truths that should be obvious.