BUG: fix .iat assignment creates a new column

doc/source/whatsnew/v0.24.0.rst

@@ -1493,6 +1493,7 @@ Indexing
 - Bug in :class:`Index` slicing with boolean :class:`Index` may raise ``TypeError`` (:issue:`22533`)
 - Bug in ``PeriodArray.__setitem__`` when accepting slice and list-like value (:issue:`23978`)
 - Bug in :class:`DatetimeIndex`, :class:`TimedeltaIndex` where indexing with ``Ellipsis`` would lose their ``freq`` attribute (:issue:`21282`)
+- Bug in ``iat`` where using it to assign an incompatible value would create a new column (:issue:`23236`)
 
 Missing
 ^^^^^^^

pandas/core/frame.py

@@ -2716,7 +2716,10 @@ def _set_value(self, index, col, value, takeable=False):
         except (KeyError, TypeError):
 
             # set using a non-recursive method & reset the cache
-            self.loc[index, col] = value
+            if takeable:
+                self.iloc[index, col] = value
+            else:
+                self.loc[index, col] = value
             self._item_cache.pop(col, None)
 
             return self

pandas/tests/indexing/test_scalar.py

@@ -198,3 +198,10 @@ def test_mixed_index_at_iat_loc_iloc_dataframe(self):
             df.at[0, 3]
         with pytest.raises(KeyError):
             df.loc[0, 3]
+
+    def test_iat_setter_incompatible_assignment(self):
+        # GH 23236
+        result = DataFrame({'a': [0, 1], 'b': [4, 5]})
+        result.iat[0, 0] = None
+        expected = DataFrame({"a": [None, 1], "b": [4, 5]})
+        tm.assert_frame_equal(result, expected)