noun. Not a real Bushism. As far as we know.
Real citation: "Touts the 'success' story of the Coast Guard’s evacuations. I suppose if you don’t consider the need for emergency evacuations themselves as a problem… Katrina: The Mother Of All Mistakertunities."
(Ana Marie Cox, "Chatology: Passing the Buck," Feb. 20, 2006, Wonkette, http://www.wonkette.com/politics/maureeen-dowd/chatology-passing-the-buck-155869.phpwonkette.com/politics/maureeen-dowd/chatology-passing-the-buck-155869.php)
Made-up citation: "Every time a mistakertunity is misunderestimated, an English teacher stabs herself with a fork."