Answer: Posterior chip fracture of the triquetrum

This is a Radiology classic. Small fragment of bone posterior to the proximal carpal row that is only seen on the lateral (look for the soft tissue swelling posteriorly at the wrist as well).

Fact: triquetral fractures are the second most common carpal bone fx (secondary of course to the scaphoid)

Reference: http://www.learningradiology.com/archives06/COW%20218-Triquetral%20fracture/triquetralfxccorrect.htm