There needs to be a short note in the text index build documentation warning the user that, if the user is calling ensureIndex to create a large text index on an existing collection, he/she needs to ensure that a sufficiently high open file descriptor limit has been set (due to the external sort). This note should include a link to http://docs.mongodb.org/manual/administration/ulimit/#recommended-settings.

SERVER-4534 is the underlying issue.