Which ideas are so dangerous they should be kept from the public?

This is a question that occupied Spinoza’s thoughts on a daily basis. He was convinced that he and his friends were not safe and had to fear for their lives if his writings would be published under his own name. He was probably right.

In countries like Belarus, Iran and North Korea, but also China and even Western Europe or the US, some ideas cause problems for those who express them. Apparently rulers regard some ideas as harmful to society. 

When this happens, does this necessarily imply censorship? Or are there actually ideas that are, now or in general, best left undiscussed? These days, isn’t it most of all a question of what ideas arise in which public arena?

A quite common idea, namely that there is no such thing as an apparent Ur-Dutchman and that national identity is per definition a historical accumulation based on internal differences and diversity, suddenly becomes a sensitive subject when uttered by crown princess Máxima. You could also say that as soon as an idea can't find an appropriate public space to be expressed in, the idea of freedom in a country is lacking.

It’s always people, not books or formulas, who overthrow power.