When updating a record in 1.4 you are no longer able to use special fields (e.g. $elemMatch) or a '.' in the field name definition.

For example

collection.update({_id: <id>, {item: {$elemMatch: {name: 'my_name'}}}}, ...)
will fail with "key $elemMatch must not start with '$'"

and

collection.update(

, ...)
will fail with "key item.name must not contain '.'"