I noticed in a lot of cartoons (mostly older, and/or european stuff) when a character was supposed to be insane, they would have him wear an inverted funnel on his head.
A little research shows that the inverted funnel has been a symbol for insanity since medieval times.
The wiki on funnels also mentions this, but i couldn't find any explanation as to the symbolism anywhere.
Here's an excerpt from an article on the painting that you linked.
"Bosch’s Stone Operation satirizes several basic chemical concepts. The funnel-hat, worn upside-down on the surgeon's head, was, then as now, an essential laboratory apparatus, and was illustrated in practical handbooks of distillation. It also appears in several of Bosch's other paintings, usually worn by sinners doomed to hellfire. Worn upside down over the surgeon’s academic hood in Bosch's scene, it marks him as an inept trickster masquerading as a university-trained scholar. The funnel appears in all of Bosch's hell-scenes, such as in the Vienna Last Judgment"
Laurinda Dixon, "Bosch's Stone Operation: meaning, medicine, and morality," Hektoen International 2 (2010): http://www.hektoeninternational.org/Boschsstoneoperation.html
