You can get into trouble with diverticulitis while lo-carbing if you allow yourself to get too constipated and find yourself straining quite a bit. The straining over and over can cause incredible pressure in your colon, causing little pockets to balloon out, which are called diverticuli. This doesn't happen overnight however, and is usually the culmination of many years of this and a predisposition to them. The condition of just having these little balloons or pockets is called diverticulosis. When the little pockets get infected, diverticulitis results. There's more to it, but that's pretty much what I remember about it.

Moral of the story: take care of your bowel - take fiber and drink plenty of water.