This is with V1.17(please refer to screenshot). On MAP31 of DOOM2.WAD, there is an HOM underneath the bars of the prison cells. (This does not occur with vanilla DOOM2, PrBoom or ZDoom; haven't tried it with others).
Since I imagine it may be asked, the machine on which this happened is a P4 2.6, 1GB RAM, Windows XP SP2, ATI Radeon 7500 mobile. I have attached a DXDiag report to this posting in case it is a graphics card problem (in a .ZIP file, eventhough only 44K, because .TXT attachments are not allowed).
This is openGL with the v1.2 model pack and the GWA files for the IWAD built with GLBSP 2.1.