The documentation is ambiguous in whether db.collection.dropIndex() can accept the "" wildcard to drop all indexes. We should probably add a note somewhere that it isn't allowed as it may drop multiple indexes if more than one exist. Users wanting to use the "" wildcard to drop all indexes should be referred to db.collection.dropIndexes() instead.
We extended the dropIndexes command in
SERVER-37727to accept an array of index names. As a result, the db.collection.dropIndex() shell helper, which is built on the dropIndexes command, now accepts multiple index names. It would be less confusing for users if we added some validation to this shell helper to reject '*' and arrays of index names.