A group of 11, usually a packaged food item or actual collection of interesting object.
The collector collects a dozen of said item, but is disillusioned by greed and decides to keep one. Antonym for 'Baker's Dozen' as in, the baker is generous and gives an extra, while the Collector's Dozen means 'to keep a portion of the lot' instead.
Hot Pocket Lover: "Man, the value pack boxes used to come with a dozen hot pockets, and now they only have 11!
Friend: "Oh yeah man, the Collector's Dozen. It's been getting more and more common nowadays."
Hot Pocket Lover: "Awh, what ever happened to the generous Baker's Dozen!"