diff --git a/src/dobjtype.cpp b/src/dobjtype.cpp
index 2243023d0c..63d4a8423c 100644
--- a/src/dobjtype.cpp
+++ b/src/dobjtype.cpp
@@ -37,6 +37,7 @@
 #include "actor.h"
 #include "templates.h"
 #include "autosegs.h"
+#include "v_text.h"
 
 TArray<PClass *> PClass::m_RuntimeActors;
 TArray<PClass *> PClass::m_Types;
@@ -185,7 +186,7 @@ void PClass::InsertIntoHash ()
 		else if (lexx == 0)
 		{ // This type has already been inserted
 		  // ... but there is no need whatsoever to make it a fatal error!
-			Printf ("Tried to register class '%s' more than once.\n", TypeName.GetChars());
+			Printf (TEXTCOLOR_RED"Tried to register class '%s' more than once.\n", TypeName.GetChars());
 			break;
 		}
 		else
diff --git a/src/info.cpp b/src/info.cpp
index 88a0ff7d2f..89628e5b7d 100644
--- a/src/info.cpp
+++ b/src/info.cpp
@@ -39,6 +39,7 @@
 #include "m_fixed.h"
 #include "c_dispatch.h"
 #include "d_net.h"
+#include "v_text.h"
 
 #include "gi.h"
 
@@ -142,15 +143,26 @@ void FActorInfo::StaticSetActorNums ()
 
 void FActorInfo::RegisterIDs ()
 {
+	const PClass *cls = PClass::FindClass(Class->TypeName);
+	bool set = false;
+
 	if (GameFilter == GAME_Any || (GameFilter & gameinfo.gametype))
 	{
 		if (SpawnID != 0)
 		{
-			SpawnableThings[SpawnID] = Class;
+			SpawnableThings[SpawnID] = cls;
+			if (cls != Class) 
+			{
+				Printf(TEXTCOLOR_RED"Spawn ID %d refers to hidden class type '%s'\n", SpawnID, cls->TypeName.GetChars());
+			}
 		}
 		if (DoomEdNum != -1)
 		{
-			DoomEdMap.AddType (DoomEdNum, Class);
+			DoomEdMap.AddType (DoomEdNum, cls);
+			if (cls != Class) 
+			{
+				Printf(TEXTCOLOR_RED"Editor number %d refers to hidden class type '%s'\n", DoomEdNum, cls->TypeName.GetChars());
+			}
 		}
 	}
 	// Fill out the list for Chex Quest with Doom's actors
@@ -158,6 +170,10 @@ void FActorInfo::RegisterIDs ()
 		(GameFilter & GAME_Doom))
 	{
 		DoomEdMap.AddType (DoomEdNum, Class, true);
+		if (cls != Class) 
+		{
+			Printf(TEXTCOLOR_RED"Editor number %d refers to hidden class type '%s'\n", DoomEdNum, cls->TypeName.GetChars());
+		}
 	}
 }
 
diff --git a/src/p_floor.cpp b/src/p_floor.cpp
index fd06ff0c1a..a03b505831 100644
--- a/src/p_floor.cpp
+++ b/src/p_floor.cpp
@@ -663,7 +663,9 @@ manual_stair:
 
 				if ( (ok = (tsec != NULL)) )
 				{
-					height += stairstep;
+					// Doom bug: Height was changed before discarding the sector as part of the stairs.
+					// Needs to be compatibility optioned because some maps (Eternall MAP25) depend on it.
+					if (i_compatflags & COMPATF_STAIRINDEX) height += stairstep;
 
 					// if sector's floor already moving, look for another
 					//jff 2/26/98 special lockout condition for retriggering
@@ -673,6 +675,9 @@ manual_stair:
 						sec = tsec;
 						continue;
 					}
+					
+					if (!(i_compatflags & COMPATF_STAIRINDEX)) height += stairstep;
+					
 				}
 				newsecnum = (int)(tsec - sectors);
 			}
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index b589e061ee..13c75eb991 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -2829,9 +2829,11 @@ void AActor::Tick ()
 	AActor *onmo;
 	int i;
 
-	assert (state != NULL);
+	//assert (state != NULL);
 	if (state == NULL)
 	{
+		Printf("Actor of type %s at (%f,%f) left without a state\n", GetClass()->TypeName.GetChars(),
+			x/65536., y/65536.);
 		Destroy();
 		return;
 	}
diff --git a/src/thingdef/thingdef_parse.cpp b/src/thingdef/thingdef_parse.cpp
index 85930b09ce..7712fe43b0 100644
--- a/src/thingdef/thingdef_parse.cpp
+++ b/src/thingdef/thingdef_parse.cpp
@@ -1072,6 +1072,12 @@ static FActorInfo *ParseActorHeader(FScanner &sc, Baggage *bag)
 		// Get actor name
 		sc.MustGetString ();
 		replaceName = sc.String;
+
+		if (replaceName == typeName)
+		{
+			sc.ScriptMessage ("Cannot replace class %s with itself", typeName.GetChars());
+			FScriptPosition::ErrorCounter++;
+		}
 	}
 
 	// Now, after the actor names have been parsed, it is time to switch to C-mode 
diff --git a/wadsrc/static/compatibility.txt b/wadsrc/static/compatibility.txt
index a4fd90659f..685fda086a 100644
--- a/wadsrc/static/compatibility.txt
+++ b/wadsrc/static/compatibility.txt
@@ -24,6 +24,11 @@ A80E7EE40E0D0C76A6FBD242BE29FE27 // map15
 	resetplayerspeed
 }
 
+6DA6FCBA8089161BDEC6A1D3F6C8D60F // Eternal Doom MAP25
+{
+	stairs
+}
+
 10E1E2B36302D31AC4AE68C84B5DC457 // Eternal Doom MAP28
 {
 	// What's really sad is that I got a separate bug report for this map