ZDoom

Voxels are clipped exactly like sprites, because that's the only way the software renderer can do it without adding something like a z-buffer.

Of course they would. GL has a depth buffer that naturally alleviates these things.

To fix it in software, you'd have to sort per-voxel, or some other suitable voxel picking algorithm.

Well they work fine in OpenGL.

I guess with the overall problems with depth clipping in the Doom engine, voxels like these may well qualify as 'don't do that'.

If a voxel object has a complex shape allowing a sprite to get "inside" it, the sprite isn't always properly clipped.

A good example would be in the [url=http://www.doomworld.com/vb/thread/63398/]voxel vehicles pack[/url] and use the chasecam around the VTOL. Here's a bunch of screenshots:
[spoiler][url=http://imgur.com/UIBYQvt][img]http://i.imgur.com/UIBYQvtl.png[/img][/url]
[url=http://imgur.com/WwCPpr6][img]http://i.imgur.com/WwCPpr6l.png[/img][/url]
[url=http://imgur.com/qNZSwDw][img]http://i.imgur.com/qNZSwDwl.png[/img][/url]
[url=http://imgur.com/HwxBADo][img]http://i.imgur.com/HwxBADol.png[/img][/url]
[url=http://imgur.com/Lxe02lN][img]http://i.imgur.com/Lxe02lNl.png[/img][/url]
[url=http://imgur.com/NmRnDEn][img]http://i.imgur.com/NmRnDEnl.png[/img][/url]
[url=http://imgur.com/eXSkAMT][img]http://i.imgur.com/eXSkAMTl.png[/img][/url]
[url=http://imgur.com/TqqCAPg][img]http://i.imgur.com/TqqCAPgl.png[/img][/url]
[url=http://imgur.com/W2Z6Enh][img]http://i.imgur.com/W2Z6Enhl.png[/img][/url]
[url=http://imgur.com/52ozTZZ][img]http://i.imgur.com/52ozTZZl.png[/img][/url]][/spoiler]

As you can see, the player sprite is seen through the engine block, but not through the rear wing. Depending on the angle, the sprite may also clip through the cockpit or not.