The documentation doesn't cover the following about the behavior of ensureIndex():

• What happens when multiple ensureIndex commands are run at the same time. (Answer: if there is no index present, the first will create the index entry in the system.indexes collection, and the rest are no-ops.)

• Noting that all ensureIndex() does is put an entry in the 'system.indexes' collection, which immediately kicks off the index build operation

• Noting that there is no way to interrupt an index build once it has begun

• Emphasising that all other operations will block while an index build is in progress