For example, when an NS file is 16MB and the blocksize is 15MB, the queryable downloading code will write out a 30MB file (the 1MB partial block gets interpreted as a full 15MB block). The first 16MB of the resulting file is correct, but the 14MB of "bonus" data corrupts the reading, in this case resulting in:

2016-12-30T15:06:17.187-0500 F STORAGE  [initandlisten] Missing system collection 'random_db.system.namespaces' for database 'random_db'
2016-12-30T15:06:17.187-0500 I -        [initandlisten] Fatal Assertion 34372 at src/mongo/db/storage/mmap_v1/mmap_v1_database_catalog_entry.cpp 475