Index names, including their full namespace (i.e. database.collection) cannot be longer than 128 characters. See the getIndexes() field name for the names of existing indexes.
Contrast this with http://docs.mongodb.org/manual/reference/limits/#Index-Name-Length:
The names of indexes, including their namespace (i.e database and collection name), cannot be longer than 125 characters. The default index name is the concatenation of the field names and index directions.
I believe the latter example is omitting the "." that would appear between the database/collection and the ".$" collection/index tokens.
Either way, we should probably discuss this limit consistently everywhere, whether that means listing the total limit or deducting the periods and explaining their presence.