Once I tried to create a jail. I don't know why, but that last time that I do that I needed to reinstall the system.
I think that was because I disconnected the device that I was using where I had the jail.
Since that, I have panic of even try to create one.
The nice thing about poudriere is that you don't need to worry about all that. It's all automated with just a few commands. I would recommend using it with ZFS though. Without ZFS it is incredibly I/O intense when it spawns new build jails.