BUG: Add note in whatsnew for DataFrame.at behavior change (pandas-dev#48324)

Author: rhshadrach

doc/source/whatsnew/v1.5.0.rst

@@ -1082,7 +1082,7 @@ Indexing
 - Bug in :meth:`DataFrame.sum` min_count changes dtype if input contains NaNs (:issue:`46947`)
 - Bug in :class:`IntervalTree` that lead to an infinite recursion. (:issue:`46658`)
 - Bug in :class:`PeriodIndex` raising ``AttributeError`` when indexing on ``NA``, rather than putting ``NaT`` in its place. (:issue:`46673`)
--
+- Bug in :meth:`DataFrame.at` would allow the modification of multiple columns (:issue:`48296`)
 
 Missing
 ^^^^^^^

pandas/tests/indexing/test_at.py

@@ -6,6 +6,8 @@
 import numpy as np
 import pytest
 
+from pandas.errors import InvalidIndexError
+
 from pandas import (
     CategoricalDtype,
     CategoricalIndex,
@@ -206,6 +208,12 @@ def test_at_frame_raises_key_error2(self, indexer_al):
         with pytest.raises(KeyError, match="^0$"):
             indexer_al(df)["a", 0]
 
+    def test_at_frame_multiple_columns(self):
+        # GH#48296 - at shouldn't modify multiple columns
+        df = DataFrame({"a": [1, 2], "b": [3, 4]})
+        with pytest.raises(InvalidIndexError, match=r"slice\(None, None, None\)"):
+            df.at[5] = [6, 7]
+
     def test_at_getitem_mixed_index_no_fallback(self):
         # GH#19860
         ser = Series([1, 2, 3, 4, 5], index=["a", "b", "c", 1, 2])