While investigating an index corruption case, I enountered the following:

The following query failed because the index pointed to a bad location. That is the issue I was investigating, not the problem I'm reporting on this ticket.

db.c.find(q).next()

This query succeeds because it is covered so avoids attempting to fetch the document at the bad location:

db.c.find(q, {_id:1}).hint({_id:1}).next()

Now attempt to determine the bad location by using showDiskLoc(); it unexpectedly fails attempting to fetch the document (verified by the stack trace), even though it should also be covered:

db.c.find(q, {_id:1}).hint({_id:1}).showDiskLoc().next()