Why don't I have a custom title by now?!
Interestingly, the Doom 3: BFG Edition "Classic Doom" has a very lame way of dealing with visplane overflows: it simply returns the last valid one, so overflows should -theoretically- result in HOM floors and ceilings, with only the very last visplane beyond the limit being rendered correctly. All other visplanes between the limit and the final one would not be rendered.
About TEXTURE* lumps: those are actually "master lists" of textures, containing only descriptions of how to build the textures and their properties. The patches (actual images) used to assemble the textures can be anywhere in the IWAD/PWADs.
However, due to limitations in how Doom works, there can only be two such lumps: TEXTURE1 (mandatory for all DOOM versions) and TEXTURE2 (mandatory for all registered/retail Doom versions, in addition to TEXTURE1). Whenever you add new textures to a PWAD, you must also include a modified TEXTURE2 lump (shareware Doom can't read it) that includes any and all textures previously included in the TEXTURE2 lump PLUS any of your own, new ones.
But, because the default contents of the TEXTURE2 lump are IWAD-specific (Ultimate Doom has one, Doom II has another etc.) you must make sure that you're using your PWAD with a compatible IWAD. You can't e.g. build your new textures assuming a Doom II IWAD, and then play it with Ultimate Doom, even if you don't use any Doom II specific textures, monsters or features.
Most ports will crash as soon as you try loading such a combination, others might "forgive" the error (but look wrong), and some like ZDoom might even try correcting it on the fly for you. Don't rely on such mechanisms, and don't release broken PWADs.
Also, as others said, you can't have multiple TEXTURE1/TEXTURE2 lumps in a PWAD: only one of them will be read, they won't be automagically merged, so it's important that The One is correct for the intended IWAD.
Last edited by Maes on 03-15-13 at 14:35