The manual (http://docs.mongodb.org/manual/administration/backups/) has the following sentence:
Before version 2.2, mhen used in combination with fsync or db.fsyncLock(), can mongod may block some reads, including those from mongodump, if there is a queued write operation waiting behind the fsync lock.

Obviously "mhen" should be "when", but even then the sentence makes little sense. I think that the word "can" should be removed.