This occurs in the context of a test that repeatedly closes databases.

<http://buildlogs.mongodb.org/build/500477d7d2a60f1426000cf5/test/500495eed2a60f6d76000a43/>

There is a known issue where the test in question might trigger a double exception (see SERVER-5101, SERVER-5724), but there's no specific evidence from the logs that double exceptions occurred in this run.