Cant't knock off Rusted Shield/Sword from Zamazenta/Zacian

src/battle_util.c

@@ -10672,6 +10672,7 @@ bool32 DoesSpeciesUseHoldItemToChangeForm(u16 species, u16 heldItemId)
             case FORM_CHANGE_BATTLE_PRIMAL_REVERSION:
             case FORM_CHANGE_BATTLE_ULTRA_BURST:
             case FORM_CHANGE_ITEM_HOLD:
+            case FORM_CHANGE_BEGIN_BATTLE:
                 if (formChanges[i].param1 == heldItemId)
                     return TRUE;
                 break;

test/battle/move_effect/knock_off.c

@@ -202,6 +202,126 @@ SINGLE_BATTLE_TEST("Knock Off doesn't knock off items from Pokemon behind substi
     } WHEN {
         TURN { MOVE(opponent, MOVE_SUBSTITUTE); MOVE(player, MOVE_KNOCK_OFF); }
     } SCENE {
-        NOT MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Poké Ball");
+        NOT MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Poké Ball!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off does knock off Mega Stones from Pokemon that don't actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET) { Item(ITEM_ABSOLITE); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Absolite!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off Mega Stones from Pokemon that actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_ABSOL) { Item(ITEM_ABSOLITE); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Absol's Absolite!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off does knock off Orbs for Primal Reversion from Pokemon that don't actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET) { Item(ITEM_RED_ORB); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Red Orb!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off Orbs for Primal Reversion from Pokemon that actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_GROUDON) { Item(ITEM_RED_ORB); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Groudon's Red Orb!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off Z-Crystals")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET) { Item(ITEM_ELECTRIUM_Z); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Electrium Z!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off Ultranecrozium Z from Pokemon that actually use it")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_NECROZMA_DUSK_MANE) { Item(ITEM_ULTRANECROZIUM_Z); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Necrozma's Ultranecrozium Z!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off does knock off other form-change hold items from Pokemon that don't actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET) { Item(ITEM_SKY_PLATE); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Sky Plate!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off other form-change hold items from Pokemon that actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_ARCEUS) { Item(ITEM_SKY_PLATE); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Arceus's Sky Plate!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off does knock off begin-battle form-change hold items from Pokemon that don't actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_WOBBUFFET) { Item(ITEM_RUSTED_SHIELD); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        MESSAGE("Wobbuffet knocked off Foe Wobbuffet's Rusted Shield!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Knock Off doesn't knock off begin-battle form-change hold items from Pokemon that actually use them")
+{
+    GIVEN {
+        PLAYER(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_ZAMAZENTA_HERO_OF_MANY_BATTLES) { Item(ITEM_RUSTED_SHIELD); }
+    } WHEN {
+        TURN { MOVE(opponent, MOVE_CELEBRATE); MOVE(player, MOVE_KNOCK_OFF); }
+    } SCENE {
+        NOT MESSAGE("Wobbuffet knocked off Foe Zamazenta's Rusted Shield!");
     }
 }