If you have Voodoo 3 and you have 16bit ambient screen, some 3D-accekerated programs/games still want a 24bit screen. Only way to fix this is to edit the sources.

Also one version of tinygl opened always 32bit screen (on which 3D wouldn't work, if you have a Voodoo board) - That problem was fixed very quickly. Well, might be better to update tinygl anyway, if you have old version