diff --git a/src/actor.h b/src/actor.h
index d1acd9ea7..60a00582f 100644
--- a/src/actor.h
+++ b/src/actor.h
@@ -1057,6 +1057,7 @@ public:
 	int				skillrespawncount;
 	int				TIDtoHate;			// TID of things to hate (0 if none)
 	FNameNoInit		Species;		// For monster families
+	TObjPtr<AActor>	alternative;	// (Un)Morphed actors stored here. Those with the MF_UNMORPHED flag are the originals.
 	TObjPtr<AActor>	tracer;			// Thing being chased/attacked for tracers
 	TObjPtr<AActor>	master;			// Thing which spawned this one (prevents mutual attacks)
 	double			Floorclip;		// value to use for floor clipping
diff --git a/src/g_shared/a_morph.cpp b/src/g_shared/a_morph.cpp
index 98af9ff25..57271f768 100644
--- a/src/g_shared/a_morph.cpp
+++ b/src/g_shared/a_morph.cpp
@@ -38,7 +38,7 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 	APlayerPawn *actor;
 
 	actor = p->mo;
-	if (actor == NULL)
+	if (actor == nullptr)
 	{
 		return false;
 	}
@@ -55,7 +55,7 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 		if ((p->mo->GetClass() == spawntype)
 			&& (p->mo->PlayerFlags & PPF_CANSUPERMORPH)
 			&& (p->morphTics < (((duration) ? duration : MORPHTICS) - TICRATE))
-			&& (p->mo->FindInventory (RUNTIME_CLASS(APowerWeaponLevel2), true) == NULL))
+			&& (p->mo->FindInventory (RUNTIME_CLASS(APowerWeaponLevel2), true) == nullptr))
 		{ // Make a super chicken
 			p->mo->GiveInventoryType (RUNTIME_CLASS(APowerWeaponLevel2));
 		}
@@ -65,7 +65,7 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 	{ // Dead players cannot morph
 		return false;
 	}
-	if (spawntype == NULL)
+	if (spawntype == nullptr)
 	{
 		return false;
 	}
@@ -94,7 +94,8 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 	}
 	morphed->Angles.Yaw = actor->Angles.Yaw;
 	morphed->target = actor->target;
-	morphed->tracer = actor;
+	morphed->tracer = actor->tracer;
+	morphed->alternative = actor;
 	morphed->FriendPlayer = actor->FriendPlayer;
 	morphed->DesignatedTeam = actor->DesignatedTeam;
 	morphed->Score = actor->Score;
@@ -113,7 +114,8 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 	morphed->flags2 |= actor->flags2 & MF2_FLY;
 	morphed->flags3 |= actor->flags3 & MF3_GHOST;
 	AActor *eflash = Spawn(((enter_flash) ? enter_flash : RUNTIME_CLASS(ATeleportFog)), actor->PosPlusZ(TELEFOGHEIGHT), ALLOW_REPLACE);
-	actor->player = NULL;
+	actor->player = nullptr;
+	actor->alternative = morphed;
 	actor->flags &= ~(MF_SOLID|MF_SHOOTABLE);
 	actor->flags |= MF_UNMORPHED;
 	actor->renderflags |= RF_INVISIBLE;
@@ -129,7 +131,7 @@ bool P_MorphPlayer (player_t *activator, player_t *p, PClassPlayerPawn *spawntyp
 	p->Vel.X = p->Vel.Y = 0;
 	morphed->ObtainInventory (actor);
 	// Remove all armor
-	for (item = morphed->Inventory; item != NULL; )
+	for (item = morphed->Inventory; item != nullptr; )
 	{
 		AInventory *next = item->Inventory;
 		if (item->IsKindOf (RUNTIME_CLASS(AArmor)))
@@ -182,7 +184,7 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
 	// because the level or game is ended while morphed,
 	// by the time it gets executed the morphed player
 	// pawn instance may have already been destroyed.
-	if (pmo == NULL || pmo->tracer == NULL)
+	if (pmo == nullptr || pmo->alternative == nullptr)
 	{
 		return false;
 	}
@@ -197,7 +199,7 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
 		return false;
 	}
 
-	mo = barrier_cast<APlayerPawn *>(pmo->tracer);
+	mo = barrier_cast<APlayerPawn *>(pmo->alternative);
 	mo->SetOrigin (pmo->Pos(), false);
 	mo->flags |= MF_SOLID;
 	pmo->flags &= ~MF_SOLID;
@@ -208,10 +210,14 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
 		player->morphTics = 2*TICRATE;
 		return false;
 	}
-	pmo->player = NULL;
+	// No longer using tracer as morph storage. That is what 'alternative' is for. 
+	// If the tracer has changed on the morph, change the original too.
+	mo->target = pmo->target;
+	mo->tracer = pmo->tracer;
+	pmo->player = nullptr;
 
 	// Remove the morph power if the morph is being undone prematurely.
-	for (AInventory *item = pmo->Inventory, *next = NULL; item != NULL; item = next)
+	for (AInventory *item = pmo->Inventory, *next = nullptr; item != nullptr; item = next)
 	{
 		next = item->Inventory;
 		if (item->IsKindOf(RUNTIME_CLASS(APowerMorph)))
@@ -252,10 +258,10 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
 	player->morphTics = 0;
 	player->MorphedPlayerClass = 0;
 	player->MorphStyle = 0;
-	player->MorphExitFlash = NULL;
+	player->MorphExitFlash = nullptr;
 	player->viewheight = mo->ViewHeight;
 	AInventory *level2 = mo->FindInventory (RUNTIME_CLASS(APowerWeaponLevel2), true);
-	if (level2 != NULL)
+	if (level2 != nullptr)
 	{
 		level2->Destroy ();
 	}
@@ -310,31 +316,31 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
 		}
 	}
 
-	AActor *eflash = NULL;
-	if (exit_flash != NULL)
+	AActor *eflash = nullptr;
+	if (exit_flash != nullptr)
 	{
 		eflash = Spawn(exit_flash, pmo->Vec3Angle(20., mo->Angles.Yaw, TELEFOGHEIGHT), ALLOW_REPLACE);
 		if (eflash)	eflash->target = mo;
 	}
 	mo->SetupWeaponSlots();		// Use original class's weapon slots.
 	beastweap = player->ReadyWeapon;
-	if (player->PremorphWeapon != NULL)
+	if (player->PremorphWeapon != nullptr)
 	{
 		player->PremorphWeapon->PostMorphWeapon ();
 	}
 	else
 	{
-		player->ReadyWeapon = player->PendingWeapon = NULL;
+		player->ReadyWeapon = player->PendingWeapon = nullptr;
 	}
 	if (correctweapon)
 	{ // Better "lose morphed weapon" semantics
 		PClassActor *morphweapon = PClass::FindActor(pmo->MorphWeapon);
-		if (morphweapon != NULL && morphweapon->IsDescendantOf(RUNTIME_CLASS(AWeapon)))
+		if (morphweapon != nullptr && morphweapon->IsDescendantOf(RUNTIME_CLASS(AWeapon)))
 		{
 			AWeapon *OriginalMorphWeapon = static_cast<AWeapon *>(mo->FindInventory (morphweapon));
-			if ((OriginalMorphWeapon != NULL) && (OriginalMorphWeapon->GivenAsMorphWeapon))
+			if ((OriginalMorphWeapon != nullptr) && (OriginalMorphWeapon->GivenAsMorphWeapon))
 			{ // You don't get to keep your morphed weapon.
-				if (OriginalMorphWeapon->SisterWeapon != NULL)
+				if (OriginalMorphWeapon->SisterWeapon != nullptr)
 				{
 					OriginalMorphWeapon->SisterWeapon->Destroy ();
 				}
@@ -344,20 +350,21 @@ bool P_UndoPlayerMorph (player_t *activator, player_t *player, int unmorphflag,
  	}
 	else // old behaviour (not really useful now)
 	{ // Assumptions made here are no longer valid
-		if (beastweap != NULL)
+		if (beastweap != nullptr)
 		{ // You don't get to keep your morphed weapon.
-			if (beastweap->SisterWeapon != NULL)
+			if (beastweap->SisterWeapon != nullptr)
 			{
 				beastweap->SisterWeapon->Destroy ();
 			}
 			beastweap->Destroy ();
 		}
 	}
-	pmo->tracer = NULL;
+	mo->alternative = nullptr;
+	pmo->alternative = nullptr;
 	pmo->Destroy ();
 	// Restore playerclass armor to its normal amount.
 	AHexenArmor *hxarmor = mo->FindInventory<AHexenArmor>();
-	if (hxarmor != NULL)
+	if (hxarmor != nullptr)
 	{
 		hxarmor->Slots[4] = mo->GetClass()->HexenArmor[0];
 	}
@@ -517,9 +524,9 @@ bool P_MorphedDeath(AActor *actor, AActor **morphed, int *morphedstyle, int *mor
 		(actor->player->morphTics) &&
 		(actor->player->MorphStyle & MORPH_UNDOBYDEATH) &&
 		(actor->player->mo) &&
-		(actor->player->mo->tracer))
+		(actor->player->mo->alternative))
 	{
-		AActor *realme = actor->player->mo->tracer;
+		AActor *realme = actor->player->mo->alternative;
 		int realstyle = actor->player->MorphStyle;
 		int realhealth = actor->health;
 		if (P_UndoPlayerMorph(actor->player, actor->player, 0, !!(actor->player->MorphStyle & MORPH_UNDOBYDEATHFORCED)))
diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 5f08f0152..a2b5275cb 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -4449,6 +4449,8 @@ enum EACSFunctions
 	-106 : KickFromGame(2),
 	*/
 
+	ACSF_CheckClass = 200,
+
 	// ZDaemon
 	ACSF_GetTeamScore = 19620,	// (int team)
 	ACSF_SetTeamScore,			// (int team, int value)
@@ -6028,6 +6030,12 @@ doplaysound:			if (funcIndex == ACSF_PlayActorSound)
 			return false;
 		}
 
+		case ACSF_CheckClass:
+		{
+			const char *clsname = FBehavior::StaticLookupString(args[0]);
+			return !!PClass::FindActor(clsname);
+		}
+
 		default:
 			break;
 	}
diff --git a/src/p_mobj.cpp b/src/p_mobj.cpp
index 8971efe25..6d8ce4418 100644
--- a/src/p_mobj.cpp
+++ b/src/p_mobj.cpp
@@ -389,6 +389,11 @@ void AActor::Serialize(FArchive &arc)
 		arc << SpriteRotation;
 	}
 
+	if (SaveVersion >= 4550)
+	{
+		arc << alternative;
+	}
+
 	{
 		FString tagstr;
 		if (arc.IsStoring() && Tag != NULL && Tag->Len() > 0) tagstr = *Tag;
@@ -1806,6 +1811,11 @@ double P_XYMovement (AActor *mo, DVector2 scroll)
 		mo->Vel.X *= fac;
 		mo->Vel.Y *= fac;
 	}
+	const double VELOCITY_THRESHOLD = 5000;	// don't let it move faster than this. Fixed point overflowed at 32768 but that's too much to make this safe.
+	if (mo->Vel.LengthSquared() >= VELOCITY_THRESHOLD*VELOCITY_THRESHOLD)
+	{
+		mo->Vel.MakeResize(VELOCITY_THRESHOLD);
+	}
 	move = mo->Vel;
 	// [RH] Carrying sectors didn't work with low speeds in BOOM. This is
 	// because BOOM relied on the speed being fast enough to accumulate
diff --git a/src/version.h b/src/version.h
index 8859cbe7d..0b2fa22e7 100644
--- a/src/version.h
+++ b/src/version.h
@@ -76,7 +76,7 @@ const char *GetVersionString();
 
 // Use 4500 as the base git save version, since it's higher than the
 // SVN revision ever got.
-#define SAVEVER 4549
+#define SAVEVER 4550
 
 #define SAVEVERSTRINGIFY2(x) #x
 #define SAVEVERSTRINGIFY(x) SAVEVERSTRINGIFY2(x)