In the Winter Bash 2016 promo YouTube video, What's Up, Doc? seems to be awarded on Meta SE.
As stated in the FAQ page, this hat is unique on Stack Overflow:
Do any sites have custom hats?
There are two hats (The NeverEnding Story and What's Up, Doc?), that can only be earned on Stack Overflow. There are two other hats (Running Ragged and Polymath) that require participation on Stack Exchange sites other than Stack Overflow.
Meta SE has Docs too? ;-)