"This Old Store" was a Cyanide & Happiness short released on the ExplosmEntertainment YouTube channel on October 13, 2016. It was deleted within an hour or two of being posted with no apparent explanation, though some people have suggested that this was due to some sort of animation error. It is unknown whether it will be restored, but it is currently the only Cyanide & Happiness short that is unavailable (unless you count a few pieces of bonus material which used to be available on Explosm's website until the show moved to Seeso).
External Link: http://explosm.net/shorts/228/this-old-store
Er...this turned out to be lost for less than a day: https://www.youtube.com/watch?v=w9R6f3TMqtM&feature=share , though the original version with the animation error is still lost.