Z_Malloc is an exit error that occurs whenever an overflow in the Doom engine (whether it be number of things, linedefs, etc.) exceeds a designated heap size for the game. The Doom engine tries to render everything but overbounds itself on its allocated memory array, and disgracefully crashes. NUTS.WAD is an example if ran on Vanilla (crash at 63047+ bytes) or Chocolate Doom (crash at 262000+ bytes).