If an update() command contains multiple $push statements, only the last one will be executed.

To reproduce:

> db.test.drop()
true
> db.test.save({_id:1, "array1":[0], "array2":["a"]})
> db.test.find()

{ "_id" : 1, "array1" : [ 0 ], "array2" : [ "a" ] }

> db.test.update({_id:1}, {$push:

{"array1":1}

, $push:{"array2":"b"}})
> db.test.find()

{ "_id" : 1, "array1" : [ 0 ], "array2" : [ "a", "b" ] }