multi-update in the presence of an error gives incorrect nModified / nMatched value

db.test.drop() db.test.ensureIndex({num:1},{unique:true}) db.test.insert([{num:0},{num:2},{num:4},{num:6},{num:7},{num:10}]) db.test.update({},{$inc:{num:1}},{multi:true}) Gives: WriteResult({ "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 11000, "errmsg" : "E11000 duplicate key error collection: mt.test index: num_1 dup key: { : 7.0 }" } }) But... > db.test.find({},{_id:0}) { "num" : 1 } { "num" : 3 } { "num" : 5 } { "num" : 6 } { "num" : 7 } { "num" : 10 } 3 documents were modified. We would expect "nMatched" and "nModified" to be at least 3.