extra smelly cheese
It may interest anyone using the "once in a blue moon" expression that it's not as uncommon as you'd suppose. Occurs roughly once every 2-3 years, with the next one due in 2007. Even more confusingly, the modern usage of the term "blue moon" is incorrect, stemming from a misinterpretation made in the 1940s.
'Blue' your misconceptions out of the water, didn't I ?
*prepares to duck under a hail of rotten vegetables*