If do a lot of inserts to a sharded collection really fast, then call getLastError, there can still be some inserts that haven't been processed yet. I have attached a test that reproduces the error, but it is very flaky. So far I haven't been able to make a test that's consistently reproducible. The test is entirely taken from sharding/auth.js, but with all other parts of the test (including all uses of authentication) removed.

http://buildbot.mongodb.org/builders/Linux%2064-bit%20Legacy/builds/3958/steps/test_3/logs/stdio has the output from a run of sharding/auth.js that failed b/c of this.