diff --git a/CHANGELOG.md b/CHANGELOG.md
index 583e52d..37dcebb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,11 @@
 - Removed
   - Python 3.6 support
   - Support for text file objects as `load` input. Use binary file objects instead.
+- Improved
+  - Raise an error when dotted keys define values outside the "current table".
+    Technically speaking TOML v1.0.0 does allow such assignments
+    but that isn't intended by specification writers,
+    and will change in a future specification version (see the [pull request](https://github.com/toml-lang/toml/pull/848)).
 
 ## 1.2.2
 
diff --git a/tests/data/extras/invalid/dotted-keys/extend-defined-aot.toml b/tests/data/extras/invalid/dotted-keys/extend-defined-aot.toml
new file mode 100644
index 0000000..1c3c34b
--- /dev/null
+++ b/tests/data/extras/invalid/dotted-keys/extend-defined-aot.toml
@@ -0,0 +1,3 @@
+[[tab.arr]]
+[tab]
+arr.val1=1
diff --git a/tests/data/extras/invalid/dotted-keys/extend-defined-table-with-subtable.toml b/tests/data/extras/invalid/dotted-keys/extend-defined-table-with-subtable.toml
new file mode 100644
index 0000000..70e2ac5
--- /dev/null
+++ b/tests/data/extras/invalid/dotted-keys/extend-defined-table-with-subtable.toml
@@ -0,0 +1,4 @@
+[a.b.c.d]
+  z = 9
+[a]
+  b.c.d.k.t = 8
diff --git a/tests/data/extras/invalid/dotted-keys/extend-defined-table.toml b/tests/data/extras/invalid/dotted-keys/extend-defined-table.toml
new file mode 100644
index 0000000..c88c179
--- /dev/null
+++ b/tests/data/extras/invalid/dotted-keys/extend-defined-table.toml
@@ -0,0 +1,4 @@
+[a.b.c]
+  z = 9
+[a]
+  b.c.t = 9
diff --git a/tests/test_flags.py b/tests/test_flags.py
deleted file mode 100644
index 64194d9..0000000
--- a/tests/test_flags.py
+++ /dev/null
@@ -1,13 +0,0 @@
-from tomli._parser import Flags
-
-
-def test_set_for_relative_key():
-    flags = Flags()
-    head_key = ("a", "b")
-    rel_key = ("c", "d")
-    flags.set_for_relative_key(head_key, rel_key, Flags.EXPLICIT_NEST)
-    assert not flags.is_(("a",), flags.EXPLICIT_NEST)
-    assert not flags.is_(("a", "b"), flags.EXPLICIT_NEST)
-    assert flags.is_(("a", "b", "c"), flags.EXPLICIT_NEST)
-    assert flags.is_(("a", "b", "c", "d"), flags.EXPLICIT_NEST)
-    assert not flags.is_(("a", "b", "c", "d", "e"), flags.EXPLICIT_NEST)
diff --git a/tomli/_parser.py b/tomli/_parser.py
index ec7c749..0cf1b23 100644
--- a/tomli/_parser.py
+++ b/tomli/_parser.py
@@ -95,6 +95,7 @@ def loads(s: str, *, parse_float: ParseFloat = float) -> dict[str, Any]:  # noqa
                 second_char: str | None = src[pos + 1]
             except IndexError:
                 second_char = None
+            out.flags.finalize_pending()
             if second_char == "[":
                 pos, header = create_list_rule(src, pos, out)
             else:
@@ -131,6 +132,15 @@ class Flags:
 
     def __init__(self) -> None:
         self._flags: dict[str, dict] = {}
+        self._pending_flags: set[tuple[Key, int]] = set()
+
+    def add_pending(self, key: Key, flag: int) -> None:
+        self._pending_flags.add((key, flag))
+
+    def finalize_pending(self) -> None:
+        for key, flag in self._pending_flags:
+            self.set(key, flag, recursive=False)
+        self._pending_flags.clear()
 
     def unset_all(self, key: Key) -> None:
         cont = self._flags
@@ -140,19 +150,6 @@ def unset_all(self, key: Key) -> None:
             cont = cont[k]["nested"]
         cont.pop(key[-1], None)
 
-    def set_for_relative_key(self, head_key: Key, rel_key: Key, flag: int) -> None:
-        cont = self._flags
-        for k in head_key:
-            if k not in cont:
-                cont[k] = {"flags": set(), "recursive_flags": set(), "nested": {}}
-            cont = cont[k]["nested"]
-        for k in rel_key:
-            if k in cont:
-                cont[k]["flags"].add(flag)
-            else:
-                cont[k] = {"flags": {flag}, "recursive_flags": set(), "nested": {}}
-            cont = cont[k]["nested"]
-
     def set(self, key: Key, flag: int, *, recursive: bool) -> None:  # noqa: A003
         cont = self._flags
         key_parent, key_stem = key[:-1], key[-1]
@@ -320,12 +317,20 @@ def key_value_rule(
     key_parent, key_stem = key[:-1], key[-1]
     abs_key_parent = header + key_parent
 
+    relative_path_cont_keys = (header + key[:i] for i in range(1, len(key)))
+    for cont_key in relative_path_cont_keys:
+        # Check that dotted key syntax does not redefine an existing table
+        if out.flags.is_(cont_key, Flags.EXPLICIT_NEST):
+            raise suffixed_err(src, pos, f"Cannot redefine namespace {cont_key}")
+        # Containers in the relative path can't be opened with the table syntax or
+        # dotted key/value syntax in following table sections.
+        out.flags.add_pending(cont_key, Flags.EXPLICIT_NEST)
+
     if out.flags.is_(abs_key_parent, Flags.FROZEN):
         raise suffixed_err(
-            src, pos, f"Can not mutate immutable namespace {abs_key_parent}"
+            src, pos, f"Cannot mutate immutable namespace {abs_key_parent}"
         )
-    # Containers in the relative path can't be opened with the table syntax after this
-    out.flags.set_for_relative_key(header, key, Flags.EXPLICIT_NEST)
+
     try:
         nest = out.data.get_or_create_nest(abs_key_parent)
     except KeyError: