I'm not exactly sure how a cleavage-butt crack comparison broke out on the set of Community but a couple of hours ago Alison Brie, Joel McHale, and Donald Glover started tweeting about it, and shortly after comparison photos were shared with their followers.
My theory (which I think Childish Gambino's tweets confirm) is this is all an elaborate ruse to trick all the Alison Brie cleavage oglers out there (if you're reading this, you're one of them) into examining dudes' butt cracks. But if you're to believe Ms. Brie she is definitely one of the two.
I've conveniently laid out the Twitter conversation (sans the sure-to-be creeptastic @ guesses) along with the photos in question. Feel free to speculate. If Donald Glover had joined in this would have been way easier. You know, because of his rumored tramp stamp. Slightly NSFW flesh-filled photos...
Relevant research material.