“It’s easy to identify people who can’t count to ten. They’re in front of you in the supermarket express line.”
June Henderson

“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”
Douglas Adams

“The stupid are deaf to the truth; they hear, but think that the wisdom applies to someone else.”
Heraclitus

“Genius may have its limitations, but stupidity is not thus handicapped." 
Elbert Green Hubbard 

“If stupidity got us into this mess, then why can’t it get us out?”
Will Rogers

The difference between stupidity and genius is that genius has its limits.
Albert Einstein

Nothing in all the world is more dangerous than sincere ignorance and conscientious stupidity.
Martin Luther King, Jr.

Stupidity combined with arrogance and a huge ego will get you a long way.
Chris Lowe

I am patient with stupidity but not with those who are proud of it.
Edith Sitwell

Never underestimate the power of human stupidity.
Robert A. Heinlein

Alcohol gives you infinite patience for stupidity.
Sammy Davis, Jr.

Never attribute to malice, that which can be reasonably explained by stupidity.
Spider Robinson

To forget one's purpose is the commonest form of stupidity.
Friedrich Nietzsche

You can never underestimate the stupidity of the general public.
Scott Adams

It is a wise man who knows where courage ends and stupidity begins.
Jerome Cady

No intelligent idea can gain general acceptance unless some stupidity is mixed in with it.
Fernando Pessoa