Responding to ext4 journal corruption

Writing garbage into the journal is quite easy, too. All it takes is for 
the disk to forget a single seek after a legitimate journal write, and 
it'll write something into the journal which was supposed to go elsewhere. 
(I've seen this on disks running live systems on ext3 for huge banks. The 
banks were not very happy, because the sysadmins simply unplugged the disk 
array after the disk errors: so the filesystem was unclean, the journal 
was replayed... and oops, that's sprayed quite a lot of garbage into the 
fs, because a multimegabyte logfile write had landed in the journal, and 
all of that was misinterpreted as metadata. That specific case, in which 
the blocks look nothing like journal blocks at all, was plugged in 
e2fsprogs 1.40, but the bank was using a version of RHEL that was still on 
1.35...)