Latest CB breaks this during stop command: http://pastie.org/7267629
Commit: Bukkit/CraftBukkit@052e955

Yeah, it's being triggered when WB tries to log the fact that it's stopping the border monitoring process.

Probably an oversight on the part of the Bukkit devs, but I'm going to add a workaround in WB shortly anyway.

After now checking it out myself, I'm not getting any errors with WorldBorder on the latest CraftBukkit dev build 2731.

I don't use JSONAPI though. The problem appears to be caused by it based on a closer look at the stacktrace you posted.