// insert a few objects like this one
> db.foo.findOne()
{ "_id" : ObjectId("52d4b437ec0439c7ad81e069"), "a" : 4 }

// multi update with $mul works on all 4
> db.foo.update({}, {$mul : {a : 2}}, false, true)
SingleWriteResult({
	"writeErrors" : [ ],
	"writeConcernErrors" : [ ],
	"nInserted" : 0,
	"nUpserted" : 0,
	"nUpdated" : 4,
	"nModified" : 4,
	"nRemoved" : 0,
	"upserted" : [ ]
})
// now insert a string
> db.foo.insert({a : "x"})
SingleWriteResult({
	"writeErrors" : [ ],
	"writeConcernErrors" : [ ],
	"nInserted" : 1,
	"nUpserted" : 0,
	"nUpdated" : 0,
	"nModified" : 0,
	"nRemoved" : 0,
	"upserted" : [ ]
})
// retry the update, expecting an error
> db.foo.update({}, {$mul : {a : 2}}, false, true)
SingleWriteResult({
	"writeErrors" : [
		[object Object]
	],
	"writeConcernErrors" : [ ],
	"nInserted" : 0,
	"nUpserted" : 0,
	"nUpdated" : 0,
	"nModified" : 0,
	"nRemoved" : 0,
	"upserted" : [ ]
})
// checking the previous error gives us the actual error
> db.getPrevError()
{
	"err" : "Cannot apply $mul to a value of non-numeric type. {_id: ObjectId('52d4b44eec0439c7ad81e06d')} has the field 'a' of non-numeric type String",
	"code" : 16837,
	"n" : 0,
	"nPrev" : 2,
	"ok" : 1
}

> db.foo.find()
{ "_id" : ObjectId("52d4b437ec0439c7ad81e069"), "a" : 64 }
{ "_id" : ObjectId("52d4b43aec0439c7ad81e06a"), "a" : 48 }
{ "_id" : ObjectId("52d4b43cec0439c7ad81e06b"), "a" : 32 }
{ "_id" : ObjectId("52d4b43dec0439c7ad81e06c"), "a" : 16 }
{ "_id" : ObjectId("52d4b44eec0439c7ad81e06d"), "a" : "x" }
{ "_id" : ObjectId("52d4b63dec0439c7ad81e06e"), "a" : "y" }
> db.foo.update({}, {$mul : {a : 2}}, false, true)
SingleWriteResult({
	"writeErrors" : [
		[object Object]
	],
	"writeConcernErrors" : [ ],
	"nInserted" : 0,
	"nUpserted" : 0,
	"nUpdated" : 0,
	"nModified" : 0,
	"nRemoved" : 0,
	"upserted" : [ ]
})
// 0 modified or updated, but if we check the values again we can see the successful modification of the 4 numeric fields
> db.foo.find()
{ "_id" : ObjectId("52d4b437ec0439c7ad81e069"), "a" : 128 }
{ "_id" : ObjectId("52d4b43aec0439c7ad81e06a"), "a" : 96 }
{ "_id" : ObjectId("52d4b43cec0439c7ad81e06b"), "a" : 64 }
{ "_id" : ObjectId("52d4b43dec0439c7ad81e06c"), "a" : 32 }
{ "_id" : ObjectId("52d4b44eec0439c7ad81e06d"), "a" : "x" }
{ "_id" : ObjectId("52d4b63dec0439c7ad81e06e"), "a" : "y" }