Just some problems issueing with /worldborder warning <everything>, also, if the border is set to the coords 0.0 0.0, (it's not a single block, it's 1/4 of 4 blocks each [at least, for me]), you will be able to go in and out.

(You can't if the border center is set like 0 0 (one block) and has 1 or 2 as size.)
/worldborder damage doesn't work too, but don't return any error. It's like an empty command.

Here you will find my pastebin log of the issue: http://pastebin.com/eRWJW1AP
P.S : This is the latest version of spigot, as said by Mr command.