This reportedly works on black cartridges only.
Enter the master cheat ''BEWAREOBLIVIONISATHAND'' and go to a deathmatch. Select options, level, and characters as normal. Enter the match. Now go to the ''Warp'' cheats and select any level to warp to. Select it TWICE, because it won't send you there the first time.
You are now in a single player level with all your multiplayer options and players! up to 4 can join the fun, but it lags badly unless you only have 2 players.Warps do not work and you cannot pick up the single player items.
Try this on level 2, and with other cheats on