There's a nasty bug in FlowView in R_Render.c.

Here's the modified version that was used in Greed:

void FlowView()
{
 entry_t *process_p, *nextprocess_p;

 process_p=entries;
 process_p->tilex=viewtilex;
 process_p->tiley=viewtiley;
 process_p->mapspot=(viewtiley<<6)+viewtilex;
 process_p->xmin=0;
 process_p->xmax=windowWidth-1;
 entry_p=process_p+1;
 memset(entrycount,0,MAPCOLS*MAPROWS*4);
 entrycounter=1;
 while (process_p<entry_p)
  {
   if (process_p->mapspot==-1) // entry has been merged
    {
     process_p++;
     continue;
     }

    /* check for mergeable entries */
   if (entrycount[process_p->mapspot]>process_p->counter) // mergeable tile
    for (nextprocess_p=process_p+1;nextprocess_p<entry_p;nextprocess_p++) // scan for mergeable entries
     if (nextprocess_p->mapspot==process_p->mapspot)
      {
       if (nextprocess_p->xmin==process_p->xmax+1) process_p->xmax=nextprocess_p->xmax;
       else if (nextprocess_p->xmax==process_p->xmin-1) process_p->xmin=nextprocess_p->xmin;
       else // bad merge!
    MS_Error("Bad tile event combination:\n"
         " nextprocess_p=%d process_p=%d\n"
         " nextprocess_p->xmin=%d  nextprocess_p->xmax=%d\n"
         " process_p->xmin=%d  process_p->xmax=%d\n",
         (int)nextprocess_p,(int)process_p,
         nextprocess_p->xmin,nextprocess_p->xmax,
         process_p->xmin,process_p->xmax);
       entrycount[nextprocess_p->mapspot]=0;
       nextprocess_p->mapspot=-1;
       }

     /* check for a dublicate entry */
   if (entrymap[process_p->mapspot]==frameon) goto end;

   entrymap[process_p->mapspot]=frameon;
   RenderTileWalls(process_p);
   RenderTileEnds();
end:
   process_p++;
   }
 }