1. Create a large dataset. 2. Bring up multiple shells. 3. In each shell perform an index creation with background:1. For example, db.myCollection.ensureIndex({a:1, d:-1}, {background:1}) // in window 1 db.myCollection.ensureIndex({e:1, c:-1}, {background:1}) // in window 2 db.myCollection.ensureIndex({a:-1, b:1}, {background:1}) // in window 3 4. The index creation in window 3 resulted in the following error. > db.myCollection.ensureIndex({a:-1, b:1}, {background:1}) { "err" : "missing Extra" , "code" : 14045, "n" : 0, "connectionId" : 2, "ok" : 1 } 5. Subsequent validate resulted in the following. > db.myCollection.validate( true ) { "ns" : "test.myCollection" , "firstExtent" : "5:191ea000 ns:test.myCollection" , "lastExtent" : "4:13c94000 ns:test.myCollection" , "extentCount" : 18, "extents" : [ { "loc" : "5:191ea000" , "xnext" : "0:f000" , "xprev" : " null " , "nsdiag" : "test.myCollection" , "size" : 8192, "firstRecord" : "5:191ea0b0" , "lastRecord" : "5:191ebf70" }, { "loc" : "0:f000" , "xnext" : "0:17000" , "xprev" : "5:191ea000" , "nsdiag" : "test.myCollection" , "size" : 32768, "firstRecord" : "0:f0b0" , "lastRecord" : "0:16f70" }, { "loc" : "0:17000" , "xnext" : "0:5b000" , "xprev" : "0:f000" , "nsdiag" : "test.myCollection" , "size" : 131072, "firstRecord" : "0:170b0" , "lastRecord" : "0:36f70" }, { "loc" : "0:5b000" , "xnext" : "0:16b000" , "xprev" : "0:17000" , "nsdiag" : "test.myCollection" , "size" : 524288, "firstRecord" : "0:5b0b0" , "lastRecord" : "0:daf70" }, { "loc" : "0:16b000" , "xnext" : "0:5ab000" , "xprev" : "0:5b000" , "nsdiag" : "test.myCollection" , "size" : 2097152, "firstRecord" : "0:16b0b0" , "lastRecord" : "0:36af70" }, { "loc" : "0:5ab000" , "xnext" : "0:16ab000" , "xprev" : "0:16b000" , "nsdiag" : "test.myCollection" , "size" : 8388608, "firstRecord" : "0:5ab0b0" , "lastRecord" : "0:daaf70" }, { "loc" : "0:16ab000" , "xnext" : "0:2178000" , "xprev" : "0:5ab000" , "nsdiag" : "test.myCollection" , "size" : 11325440, "firstRecord" : "0:16ab0b0" , "lastRecord" : "0:2177f70" }, { "loc" : "0:2178000" , "xnext" : "1:2000" , "xprev" : "0:16ab000" , "nsdiag" : "test.myCollection" , "size" : 15290368, "firstRecord" : "0:21780b0" , "lastRecord" : "0:300cf50" }, { "loc" : "1:2000" , "xnext" : "1:13b2000" , "xprev" : "0:2178000" , "nsdiag" : "test.myCollection" , "size" : 20643840, "firstRecord" : "1:20b0" , "lastRecord" : "1:13b1f90" }, { "loc" : "1:13b2000" , "xnext" : "1:3eae000" , "xprev" : "1:2000" , "nsdiag" : "test.myCollection" , "size" : 27869184, "firstRecord" : "1:13b20b0" , "lastRecord" : "1:2e45f90" }, { "loc" : "1:3eae000" , "xnext" : "2:2000" , "xprev" : "1:13b2000" , "nsdiag" : "test.myCollection" , "size" : 37625856, "firstRecord" : "1:3eae0b0" , "lastRecord" : "1:628ff90" }, { "loc" : "2:2000" , "xnext" : "2:469a000" , "xprev" : "1:3eae000" , "nsdiag" : "test.myCollection" , "size" : 50798592, "firstRecord" : "2:20b0" , "lastRecord" : "2:3073f90" }, { "loc" : "2:469a000" , "xnext" : "2:a5e8000" , "xprev" : "2:2000" , "nsdiag" : "test.myCollection" , "size" : 68579328, "firstRecord" : "2:469a0b0" , "lastRecord" : "2:8800f90" }, { "loc" : "2:a5e8000" , "xnext" : "3:2861000" , "xprev" : "2:469a000" , "nsdiag" : "test.myCollection" , "size" : 92581888, "firstRecord" : "2:a5e80b0" , "lastRecord" : "2:fe32f50" }, { "loc" : "3:2861000" , "xnext" : "3:d614000" , "xprev" : "2:a5e8000" , "nsdiag" : "test.myCollection" , "size" : 124985344, "firstRecord" : "3:28610b0" , "lastRecord" : "3:9f92f50" }, { "loc" : "3:d614000" , "xnext" : "4:2000" , "xprev" : "3:2861000" , "nsdiag" : "test.myCollection" , "size" : 168730624, "firstRecord" : "3:d6140b0" , "lastRecord" : "3:176fdf50" }, { "loc" : "4:2000" , "xnext" : "4:13c94000" , "xprev" : "3:d614000" , "nsdiag" : "test.myCollection" , "size" : 227786752, "firstRecord" : "4:20b0" , "lastRecord" : "4:d93df50" }, { "loc" : "4:13c94000" , "xnext" : " null " , "xprev" : "4:2000" , "nsdiag" : "test.myCollection" , "size" : 307515392, "firstRecord" : "4:13c940b0" , "lastRecord" : "4:19e6df30" } ], "datasize" : 800000080, "nrecords" : NumberLong( "4379697861059674116" ), "lastExtentSize" : 0, "padding" : 1, "firstExtentDetails" : { "loc" : "5:191ea000" , "xnext" : "0:f000" , "xprev" : " null " , "nsdiag" : "test.myCollection" , "size" : 8192, "firstRecord" : "5:191ea0b0" , "lastRecord" : "5:191ebf70" }, "lastExtentDetails" : { "loc" : "4:13c94000" , "xnext" : " null " , "xprev" : "4:2000" , "nsdiag" : "test.myCollection" , "size" : 307515392, "firstRecord" : "4:13c940b0" , "lastRecord" : "4:19e6df30" }, "objectsFound" : 10000000, "invalidObjects" : 0, "bytesWithHeaders" : 960000080, "bytesWithoutHeaders" : 800000080, "deletedCount" : 13, "deletedSize" : 204911440, "nIndexes" : 16896, "valid" : false , "errors" : [ "exception during index validate idxn 4" ], "advice" : "ns corrupt, requires repair" , "ok" : 1 } The log file is attached to the ticket. Please note, there were multiple attempts to drop another index on the same collection. That did not seem to cause any issues. Only when the third index creation was kicked off, we observed the error in step 4.