One thing that humbles me deeply is to see that human genius has its limits while human stupidity does not.

[Une chose qui m’humilie profondément est de voir que le génie humain a des limites, quand la bêtise humaine n’en a pas.]

Alexandre Dumas, fils (1824-1895) French writer and dramatist

(Attributed)
    (Source)

Earliest attribution is in the Great Universal Dictionary of the Nineteenth Century [Grand Dictionnaire Universel du XIXe Siècle], Vol. 2, "Stupidity [Bêtise]" (c. 1865) 

Attributed to a wide variety of individuals, including (spuriously) to Albert Einstein.

Variants:

• "What distresses me is to see that human genius has limitations, and human stupidity has none."
• "How despairing it is to see that human genius has limitations, while human stupidity has none."
• "The difference between genius and stupidity is that genius has its limits."
• "Human genius has its limits, but stupidity does not."
• "Genius may have its limitations, but stupidity is not thus handicapped." (Elbert Hubbard, ed., The Philistine, title epigraph (Sep 1906)

See here for more discussion.