A player killing another with World has only happened 3 times total and it's been the same each time: a medic killing a HW with World right after the HW kills the medic.
Code:
L 04/22/2013 - 23:46:25: "scrumptious. homie'<73><STEAM_0:1:3700597><#FF_TEAM_RED>" killed "zexel?<71><STEAM_0:0:15890560><#FF_TEAM_BLUE>" with "weapon_assaultcannon"
L 04/22/2013 - 23:46:27: "zexel?<71><STEAM_0:0:15890560><#FF_TEAM_BLUE>" killed "scrumptious. homie'<73><STEAM_0:1:3700597><#FF_TEAM_RED>" with "world"
L 04/22/2013 - 23:50:01: "scrumptious. homie'<73><STEAM_0:1:3700597><#FF_TEAM_RED>" killed "zexel?<71><STEAM_0:0:15890560><#FF_TEAM_BLUE>" with "weapon_assaultcannon"
L 04/22/2013 - 23:50:02: "zexel?<71><STEAM_0:0:15890560><#FF_TEAM_BLUE>" killed "scrumptious. homie'<73><STEAM_0:1:3700597><#FF_TEAM_RED>" with "world"
L 02/05/2013 - 21:08:15: "teamcake. NeoNLight*pS+M|M+<61><STEAM_0:0:3610613><#FF_TEAM_RED>" killed "BOSS | MafiaMan^HeLL<65><STEAM_0:0:138099><#FF_TEAM_BLUE>" with "weapon_assaultcannon"
L 02/05/2013 - 21:08:15: "BOSS | MafiaMan^HeLL<65><STEAM_0:0:138099><#FF_TEAM_BLUE>" killed "teamcake. NeoNLight*pS+M|M+<61><STEAM_0:0:3610613><#FF_TEAM_RED>" with "world"
I think what's happening is that the medic is infecting the HW, then the HW kills the medic. The HW then types kill in console (or changes team; anything that kills the HW). When you suicide while infected, the infected player gets the kill. But, the key is that the medic is dead; it tries to use the medic's active weapon as the killing weapon, but none exists, so it defaults to World.
That's my theory.