Multi update doesn't affect all matching documents. As far as I can tell, the bug only occurs when the query is able to use an index.

This mongo shell transcript demonstrates the problem:

> db.foo.drop();
true
> db.foo.ensureIndex(

{k:1}

)
> for (i=0; i<10; i++) { db.foo.save(

{k: 'x', a: []}

); }
> db.foo.find();

{ "_id" : ObjectId("4bd85f77ad41955b0dcda4b7"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4b8"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4b9"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4ba"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bb"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bc"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bd"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4be"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bf"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4c0"), "k" : "x", "a" : [ ] }

> db.foo.update(

{k: 'x'}

, {$push: {a: 'y'}}, false, true);
> db.foo.find();

{ "_id" : ObjectId("4bd85f77ad41955b0dcda4b8"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4ba"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bc"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4be"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4c0"), "k" : "x", "a" : [ ] } { "_id" : ObjectId("4bd85f77ad41955b0dcda4b7"), "a" : [ "y" ], "k" : "x" } { "_id" : ObjectId("4bd85f77ad41955b0dcda4b9"), "a" : [ "y" ], "k" : "x" } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bb"), "a" : [ "y" ], "k" : "x" } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bd"), "a" : [ "y" ], "k" : "x" } { "_id" : ObjectId("4bd85f77ad41955b0dcda4bf"), "a" : [ "y" ], "k" : "x" }

>

If I omit the ensureIndex call, all of the documents are updated correctly.

Tested in 1.2.2, 1.4.0, 1.4.2, and the most recent nightly dev build (v1.5.1-pre).