BUG: OverflowError when fillna on DataFrame with a pd.Timestamp (#61208)

doc/source/whatsnew/v3.0.0.rst

@@ -644,6 +644,7 @@ Datetimelike
 - Bug in :func:`date_range` where using a negative frequency value would not include all points between the start and end values (:issue:`56147`)
 - Bug in :func:`tseries.api.guess_datetime_format` would fail to infer time format when "%Y" == "%H%M" (:issue:`57452`)
 - Bug in :func:`tseries.frequencies.to_offset` would fail to parse frequency strings starting with "LWOM" (:issue:`59218`)
+- Bug in :meth:`DataFrame.fillna` raising an ``AssertionError`` instead of ``OutOfBoundsDatetime`` when filling a ``datetime64[ns]`` column with an out-of-bounds timestamp. Now correctly raises ``OutOfBoundsDatetime``. (:issue:`61208`)
 - Bug in :meth:`DataFrame.min` and :meth:`DataFrame.max` casting ``datetime64`` and ``timedelta64`` columns to ``float64`` and losing precision (:issue:`60850`)
 - Bug in :meth:`Dataframe.agg` with df with missing values resulting in IndexError (:issue:`58810`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` does not raise on Custom business days frequencies bigger then "1C" (:issue:`58664`)

pandas/core/internals/blocks.py

@@ -1749,6 +1749,8 @@ def putmask(self, mask, new) -> list[Block]:
         try:
             # Caller is responsible for ensuring matching lengths
             values._putmask(mask, new)
+        except OutOfBoundsDatetime:
+            raise
         except (TypeError, ValueError):
             if self.ndim == 1 or self.shape[0] == 1:
                 if isinstance(self.dtype, IntervalDtype):

pandas/tests/frame/methods/test_fillna.py

@@ -1,6 +1,8 @@
 import numpy as np
 import pytest
 
+from pandas.errors import OutOfBoundsDatetime
+
 from pandas import (
     Categorical,
     DataFrame,
@@ -781,3 +783,15 @@ def test_fillna_with_none_object(test_frame, dtype):
     if test_frame:
         expected = expected.to_frame()
     tm.assert_equal(result, expected)
+
+
+def test_fillna_out_of_bounds_datetime():
+    # GH#61208
+    df = DataFrame(
+        {"datetime": date_range("1/1/2011", periods=3, freq="h"), "value": [1, 2, 3]}
+    )
+    df.iloc[0, 0] = None
+
+    msg = "Cannot cast 0001-01-01 00:00:00 to unit='ns' without overflow"
+    with pytest.raises(OutOfBoundsDatetime, match=msg):
+        df.fillna(Timestamp("0001-01-01"), inplace=True)