Exploding Kittens The Hidden Pack

It might be widely available now, but the hidden pack was a collaboration between Cards Against Humanity and Exploding Kittens and could be found at Target locations by looking in the display cardboard that was set-up at the time.

It only contains 1 card that was released with this pack called the Blind Bat card. This pack came with the party pack back as well as the original card back so they could be used for both versions of the game. 

This card served as the precursor to the Curse of Cat Butt released in a later set and seemed to be the least favored card created for the game by its creators and some of the loyal Exploding Kittens fans because they feel it adds complexity to the game.