D -- The Spine
E -- The Crotch
Every climber must work to avoid crossloading carabiners. Pay attention to how the carabiner hangs off your harness while belaying or rappelling. Use your belay loop to ensure that the load is on the spine. And watch for situations where the gate to a carbiner might be compromised.
Carabiner gates present the most common problem. In the alpine -- especially in a crevasse rescue situation -- snow may get caught in between the nose and the gate allowing for the carabiner to be open at a nearly imperceptable margin. On rock, a carabiner facing the wall may be pushed open by a protrusion. Each of these problems are solved by careful recognition of the possiblity of a problem. Double check everything and use locking carabiners when possible.