I and my cousins were playing Alan Bond's "Netwars" levelset
http://www.zomis.net/rnd/download.php?id=339
However, whenever one of us died it said "Internal error" or something and then we could not continue the level. I guess this is because the player is recreated (because of the 9 lives available) and this doesn't work in Network playing apparently.
Multiplayer - Alan Bond's Netwars
Moderators: Flumminator, Zomis
Rocks'n'Diamonds 3.2.4 SDL for 32-bit Windows NT (XP, Vista)
I have seen LAN player teleportation cause other players who were not involved to have their view centered on some other place, and then the player can appear to walk off of the side of the level based on the view. But you can't see the player in the view.
The H. World levelset can be downloaded from http://www.bd-fans.com/RnD.html -- search The H. World on that page.