In a dev environement on RHEL 6.3, I am running mash (latest git) every 5 minutes against 20 koji tags.(both 1.6 and 1.7)  
I am running sequentially the "mash tag" 
After few hours my rpmdb is corrupted and I need to execute db_recover -h /var/lib/rpm.
It seems "createrepo" (0.9.8-5.el6) doesn't release the locks correctly everytime. (/usr/lib/rpm/rpmdb_stat -CA)
Does someone already ran into this issue before I investigate more ?