I like to think that I 'know' Perl. I also 'know' C, Java, Smalltalk, Ruby and a handful of other languages. But Lisp is the only one that continually makes me feel like a newbie. Just when you think you've got it pegged... BLAM— you read something that shows you a completely new dimension.
A good example of this for me is Nikodemus Siivola's post on "Optimizing Lookup Functions Using LOAD-TIME-VALUE", basically demonstrating a way of altering the language such that hash table lookups with keys that are known to be constant at compile time have no lookup overhead at runtime.
I feel like I have a whole lot to learn, and it's good :-)