I noticed a problem with my pvp area yesterday when two other players could not damage each other with swords or fists. They could only damage each other using bows. The asked a moderator to check it out (EtherealSquid). The weird thing about this was they could damage EtherealSquid and I in the pvp area with swords/fists. I'm not sure if this an issue with the pvp region that was created or something with permissions. The pvp area is at /warp pixeltown.
Tested out with @PixelGuy75 in the arena and we were able to damage each other in combat. Users in question probably were in the same party when they tried to fight, hence why no damage was inflicted.