WriteConflict returned to client with concurrent multikey index inserts

We have a handful of threads doing inserts and finds against a collection with a multikey index. The inserts should succeed, but the WriteResult returned by the server looks like this:

{

    "nInserted" : 0,

    "writeError" : {

        "code" : 112,

        "errmsg" : "WriteConflict"

    }

}

The bug can happen with as few as 2 threads, but it is more reliable to reproduce with ~8 threads. Running with dozens or hundreds of threads seems to make it less reliable to reproduce.