BUG: Edited documentation and improved tests pandas-dev#46071

Author: KingOfTheShow69

pandas/core/tools/datetimes.py

@@ -796,7 +796,10 @@ def to_datetime(
         If :const:`True` and no `format` is given, attempt to infer the format
         of the datetime strings based on the first non-NaN element,
         and if it can be inferred, switch to a faster method of parsing them.
-        In some cases this can increase the parsing speed by ~5-10x.
+        In some cases this can increase the parsing speed by ~5-10x. If subsequent
+        datetime strings do not follow the inferred format, parsing will fall
+        back to the slower method of determining the format for each
+        string individually.
     origin : scalar, default 'unix'
         Define the reference date. The numeric values would be parsed as number
         of units (defined by `unit`) since this reference date.

pandas/tests/arrays/test_datetimes.py

@@ -3,11 +3,13 @@
 """
 import operator
 
+from dateutil.parser._parser import ParserError
 import numpy as np
 import pytest
 
 from pandas._libs.tslibs import tz_compare
 from pandas._libs.tslibs.dtypes import NpyDatetimeUnit
+from pandas.errors import OutOfBoundsDatetime
 
 from pandas.core.dtypes.dtypes import DatetimeTZDtype
 
@@ -657,3 +659,22 @@ def test_coerce_fallback(self, error):
             assert result[1] is not pd.NaT
 
         tm.assert_series_equal(expected, result)
+
+        expected2 = pd.Series([pd.Timestamp("2000-01-01 00:00:00"), pd.NaT])
+
+        es1 = pd.Series(["1/1/2000", "7/12/1200"])
+        es2 = pd.Series(["1/1/2000", "Hello"])
+
+        if error == "coerce":
+            eres1 = pd.to_datetime(es1, errors=error, infer_datetime_format=True)
+            eres2 = pd.to_datetime(es2, errors=error, infer_datetime_format=True)
+            tm.assert_series_equal(expected2, eres1)
+            tm.assert_series_equal(expected2, eres2)
+        else:
+            with pytest.raises(
+                OutOfBoundsDatetime, match="Out of bounds nanosecond timestamp"
+            ):
+                pd.to_datetime(es1, errors=error, infer_datetime_format=True)
+
+            with pytest.raises(ParserError, match="Unknown string format: Hello"):
+                pd.to_datetime(es2, errors=error, infer_datetime_format=True)