diff --git a/builtin/game/register.lua b/builtin/game/register.lua
index dc4aa1ca1..2417a1b8a 100644
--- a/builtin/game/register.lua
+++ b/builtin/game/register.lua
@@ -129,6 +129,13 @@ function core.register_entity(name, prototype)
 	prototype.mod_origin = core.get_current_modname() or "??"
 end
 
+local default_tables = {
+	node = core.nodedef_default,
+	craft = core.craftitemdef_default,
+	tool = core.tooldef_default,
+	none = core.noneitemdef_default,
+}
+
 function core.register_item(name, itemdef)
 	-- Check name
 	if name == nil then
@@ -140,13 +147,6 @@ function core.register_item(name, itemdef)
 	end
 	itemdef.name = name
 
-	local mt = getmetatable(itemdef)
-	if mt ~= nil and next(mt) ~= nil then
-		core.log("warning", "Item definition has a metatable, this is "..
-			"unsupported and it will be overwritten: " .. name)
-	end
-
-	-- Apply defaults and add to registered_* table
 	if itemdef.type == "node" then
 		-- Use the nodebox as selection box if it's not set manually
 		if itemdef.drawtype == "nodebox" and not itemdef.selection_box then
@@ -162,19 +162,27 @@ function core.register_item(name, itemdef)
 			core.log("warning", "Node 'light_source' value exceeds maximum," ..
 				" limiting to maximum: " ..name)
 		end
-		setmetatable(itemdef, {__index = core.nodedef_default})
-		core.registered_nodes[itemdef.name] = itemdef
-	elseif itemdef.type == "craft" then
-		setmetatable(itemdef, {__index = core.craftitemdef_default})
-		core.registered_craftitems[itemdef.name] = itemdef
-	elseif itemdef.type == "tool" then
-		setmetatable(itemdef, {__index = core.tooldef_default})
-		core.registered_tools[itemdef.name] = itemdef
-	elseif itemdef.type == "none" then
-		setmetatable(itemdef, {__index = core.noneitemdef_default})
-	else
+	end
+
+	-- Apply defaults
+	local defaults = default_tables[itemdef.type]
+	if defaults == nil then
 		error("Unable to register item: Type is invalid: " .. dump(itemdef))
 	end
+	local old_mt = getmetatable(itemdef)
+	-- TODO most of these checks should become an error after a while (maybe in 2026?)
+	if old_mt ~= nil and next(old_mt) ~= nil then
+		-- Note that even registering multiple identical items with the same table
+		-- is not allowed, due to the 'name' property.
+		if old_mt.__index == defaults then
+			core.log("warning", "Item definition table was reused between registrations. "..
+				"This is unsupported and broken: " .. name)
+		else
+			core.log("warning", "Item definition has a metatable, this is "..
+				"unsupported and it will be overwritten: " .. name)
+		end
+	end
+	setmetatable(itemdef, {__index = defaults})
 
 	-- Flowing liquid uses param2
 	if itemdef.type == "node" and itemdef.liquidtype == "flowing" then
@@ -204,9 +212,17 @@ function core.register_item(name, itemdef)
 	-- Ignore new keys as a failsafe to prevent mistakes
 	getmetatable(itemdef).__newindex = function() end
 
-	--core.log("Registering item: " .. itemdef.name)
+	-- Add to registered_* tables
+	if itemdef.type == "node" then
+		core.registered_nodes[itemdef.name] = itemdef
+	elseif itemdef.type == "craft" then
+		core.registered_craftitems[itemdef.name] = itemdef
+	elseif itemdef.type == "tool" then
+		core.registered_tools[itemdef.name] = itemdef
+	end
 	core.registered_items[itemdef.name] = itemdef
 	core.registered_aliases[itemdef.name] = nil
+
 	register_item_raw(itemdef)
 end
 
@@ -217,17 +233,11 @@ function core.unregister_item(name)
 		return
 	end
 	-- Erase from registered_* table
-	local type = core.registered_items[name].type
-	if type == "node" then
-		core.registered_nodes[name] = nil
-	elseif type == "craft" then
-		core.registered_craftitems[name] = nil
-	elseif type == "tool" then
-		core.registered_tools[name] = nil
-	end
+	core.registered_nodes[name] = nil
+	core.registered_craftitems[name] = nil
+	core.registered_tools[name] = nil
 	core.registered_items[name] = nil
 
-
 	unregister_item_raw(name)
 end
 
diff --git a/doc/lua_api.md b/doc/lua_api.md
index f0d9624d0..765a1d31b 100644
--- a/doc/lua_api.md
+++ b/doc/lua_api.md
@@ -5895,16 +5895,23 @@ Call these functions only at load time!
 
 ### Environment
 
-* `core.register_node(name, node definition)`
-* `core.register_craftitem(name, item definition)`
-* `core.register_tool(name, item definition)`
+* `core.register_node(name, nodedef)`
+    * register a node with its definition
+    * Note: you must pass a clean table that hasn't already been used for
+      another registration to this function, as it will be modified.
+* `core.register_craftitem(name, itemdef)`
+    * register an item with its definition
+    * Note: (as above)
+* `core.register_tool(name, tooldef)`
+    * register a tool item with its definition
+    * Note: (as above)
 * `core.override_item(name, redefinition, del_fields)`
     * `redefinition` is a table of fields `[name] = new_value`,
       overwriting fields of or adding fields to the existing definition.
     * `del_fields` is a list of field names to be set
       to `nil` ("deleted from") the original definition.
     * Overrides fields of an item registered with register_node/tool/craftitem.
-    * Note: Item must already be defined, (opt)depend on the mod defining it.
+    * Note: Item must already be defined.
     * Example: `core.override_item("default:mese",
       {light_source=core.LIGHT_MAX}, {"sounds"})`:
       Overwrites the `light_source` field,
@@ -5912,7 +5919,7 @@ Call these functions only at load time!
 * `core.unregister_item(name)`
     * Unregisters the item from the engine, and deletes the entry with key
       `name` from `core.registered_items` and from the associated item table
-      according to its nature: `core.registered_nodes`, etc.
+      according to its nature (e.g. `core.registered_nodes`)
 * `core.register_entity(name, entity definition)`
 * `core.register_abm(abm definition)`
 * `core.register_lbm(lbm definition)`