Hello,
Someone this bug reported in my Interlude L2JFree server. :)
Ok, just go outside city in normal zone and make SOE or /unstuck to city after tp dont move and server think, what your still are not in city, and you can attack other players like outside city.
If you make tp from PVP zone (arena, olympiad) you can kill others players in city without karma! Just dont move after teleport!