Compatibility and Index Type Changes in MongoDB 2.4
In 2.4 MongoDB includes two new features related to indexes that users upgrading to version 2.4 must consider, particularly with regard to possible downgrade paths. For more information on downgrades, see Downgrade MongoDB from 2.4 to Previous Versions.
In 2.4 MongoDB adds two new index types:
text. These index types do not exist in 2.2, and for each database, creating a
text index, will upgrade the data-file version and make that database incompatible with 2.2.
If you intend to downgrade, you should always drop all
text indexes before moving to 2.2.
In MongoDB 2.2 and earlier you could specify invalid index types that did not exist. In these situations, MongoDB would create an ascending (e.g.
1) index. Invalid indexes include index types specified by strings that do not refer to an existing index type, and all numbers other than
In 2.4, creating any invalid index will result in an error. Furthermore, you cannot create a
text index on a collection if its containing database has any invalid index types. 
If you attempt to add an invalid index in MongoDB 2.4, as in the following:
MongoDB will return the following error document:
|||(1, 2) In 2.4, indexes that specify a type of