The following sequence leaves the namespace in an inconsistent/broken state:
- format NN using 0.20 (or any prior release, probably)
- run hdfs namenode -upgrade on 0.22. ^C the NN once it comes up.
- run hdfs namenode -rollback on 0.22 (this should fail but doesn't!)
This leaves the name directory in a state such that the version file claims it's an 0.20 namespace, but the fsimage is in 0.22 format. It then crashes when trying to start up.