diff --git a/src/compatibility.cpp b/src/compatibility.cpp
index 880f887899..bfd5ae3257 100644
--- a/src/compatibility.cpp
+++ b/src/compatibility.cpp
@@ -90,6 +90,7 @@ static FCompatOption Options[] =
 	{ "resetplayerspeed",		0, BCOMPATF_RESETPLAYERSPEED },
 	{ "vileghosts",				0, BCOMPATF_VILEGHOSTS },
 	{ "ignoreteleporttags",		0, BCOMPATF_BADTELEPORTERS },
+	{ "oldportals",				0, BCOMPATF_BADPORTALS },
 
 	// list copied from g_mapinfo.cpp
 	{ "shorttex",				COMPATF_SHORTTEX, 0 },
@@ -280,7 +281,12 @@ void CheckCompatibility(MapData *map)
 		ib_compatflags = 0;
 		ii_compatparams = -1;
 	}
-
+	else if (Wads.GetLumpFile(map->lumpnum) == 1 && (gameinfo.flags & GI_COMPATPORTAL))
+	{
+		ii_compatflags = 0;
+		ib_compatflags = BCOMPATF_BADPORTALS;
+		ii_compatparams = -1;
+	}
 	else
 	{
 		map->GetChecksum(md5.Bytes);
diff --git a/src/d_iwad.cpp b/src/d_iwad.cpp
index 6fd42a6292..123f33f7d4 100644
--- a/src/d_iwad.cpp
+++ b/src/d_iwad.cpp
@@ -174,6 +174,7 @@ void FIWadManager::ParseIWadInfo(const char *fn, const char *data, int datasize)
 						else if(sc.Compare("Extended")) iwad->flags |= GI_MENUHACK_EXTENDED;
 						else if(sc.Compare("Shorttex")) iwad->flags |= GI_COMPATSHORTTEX;
 						else if(sc.Compare("Stairs")) iwad->flags |= GI_COMPATSTAIRS;
+						else if(sc.Compare("Portals")) iwad->flags |= GI_BADPORTALS;
 						else sc.ScriptError(NULL);
 					}
 					while (sc.CheckString(","));
diff --git a/src/doomdef.h b/src/doomdef.h
index cad56d642c..748f2df4a9 100644
--- a/src/doomdef.h
+++ b/src/doomdef.h
@@ -342,6 +342,7 @@ enum
 	BCOMPATF_RESETPLAYERSPEED	= 1 << 1,	// Set player speed to 1.0 when changing maps
 	BCOMPATF_VILEGHOSTS			= 1 << 2,	// Monsters' radius and height aren't restored properly when resurrected.
 	BCOMPATF_BADTELEPORTERS		= 1 << 3,	// Ignore tags on Teleport specials
+	BCOMPATF_BADPORTALS			= 1 << 4,	// Restores the old unstable portal behavior
 };
 
 // phares 3/20/98:
diff --git a/src/p_spec.cpp b/src/p_spec.cpp
index eeb58d0c36..a2e09d347b 100644
--- a/src/p_spec.cpp
+++ b/src/p_spec.cpp
@@ -975,6 +975,9 @@ void P_SetupPortals()
 		points.Push(pt);
 	}
 
+	// Maps using undefined portal hacks may not benefit from portal optimizations.
+	if (ib_compatflags & BCOMPATF_BADPORTALS) return;
+
 	for(unsigned i=0;i<points.Size(); i++)
 	{
 		if (points[i]->special1 == 0 && points[i]->Mate != NULL)
diff --git a/src/r_plane.cpp b/src/r_plane.cpp
index eaa3d4b471..3fa6bdf73f 100644
--- a/src/r_plane.cpp
+++ b/src/r_plane.cpp
@@ -582,20 +582,27 @@ visplane_t *R_FindPlane (const secplane_t &height, FTextureID picnum, int lightl
 						check->viewx == stacked_viewx &&
 						check->viewy == stacked_viewy &&
 						check->viewz == stacked_viewz &&
-						check->alpha == alpha &&
-						(alpha == 0 ||	// if alpha is > 0 everything needs to be checked
-							(plane == check->height &&
-							 picnum == check->picnum &&
-							 lightlevel == check->lightlevel &&
-							 xoffs == check->xoffs &&	// killough 2/28/98: Add offset checks
-							 yoffs == check->yoffs &&
-							 basecolormap == check->colormap &&	// [RH] Add more checks
-							 xscale == check->xscale &&
-							 yscale == check->yscale &&
-							 angle == check->angle
+						(
+							// headache inducing logic... :(
+							(ib_compatflags & BCOMPATF_BADPORTALS) ||
+							(
+								check->alpha == alpha &&
+								(alpha == 0 ||	// if alpha is > 0 everything needs to be checked
+									(plane == check->height &&
+									 picnum == check->picnum &&
+									 lightlevel == check->lightlevel &&
+									 xoffs == check->xoffs &&	// killough 2/28/98: Add offset checks
+									 yoffs == check->yoffs &&
+									 basecolormap == check->colormap &&	// [RH] Add more checks
+									 xscale == check->xscale &&
+									 yscale == check->yscale &&
+									 angle == check->angle
+									)
+								) &&
+								check->viewangle == stacked_angle
 							)
-						) &&
-						check->viewangle == stacked_angle)
+						)
+					   )
 					{
 						return check;
 					}
diff --git a/wadsrc/static/iwadinfo.txt b/wadsrc/static/iwadinfo.txt
index efdf836a59..7a94f244ad 100644
--- a/wadsrc/static/iwadinfo.txt
+++ b/wadsrc/static/iwadinfo.txt
@@ -26,6 +26,7 @@ IWad
 	Game = "Doom"
 	Config = "UrbanBrawl"
 	Mapinfo = "mapinfo/urbanbrawl.txt"
+	Compatibility = "Portals"
 	MustContain = "MAP01", "AD2LIB"
 	BannerColors = "a8 a8 00", "a8 00 00"
 }