BUG: inserting list of strings into Series auto-infers them as datetimes with mixed formats

[MarcoGorelli]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

In [3]: df = pd.DataFrame({'a': pd.date_range('2000', freq='D', periods=2)})

In [4]: df.loc[:, 'a'] = ['12/01/2020', '13/01/2020']

In [5]: df
Out[5]:
           a
0 2020-12-01
1 2020-01-13

Issue Description

Similar to https://pandas.pydata.org/pdeps/0004-consistent-to-datetime-parsing.html

Expected Behavior

I think that inferring strings as datetimes is fine so long as they're parsed in a consistent format

Installed Versions

INSTALLED VERSIONS

commit : 57fd502
python : 3.10.12
python-bits : 64
OS : Linux
OS-release : 5.15.167.4-microsoft-standard-WSL2
Version : #1 SMP Tue Nov 5 00:21:55 UTC 2024
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.UTF-8
LOCALE : en_US.UTF-8

pandas : 3.0.0.dev0+1979.g57fd50221e
numpy : 1.26.4
dateutil : 2.9.0.post0
pip : 25.0.1
Cython : 3.0.12
sphinx : 8.1.3
IPython : 8.33.0
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.3
blosc : None
bottleneck : 1.4.2
fastparquet : 2024.11.0
fsspec : 2025.2.0
html5lib : 1.1
hypothesis : 6.127.5
gcsfs : 2025.2.0
jinja2 : 3.1.5
lxml.etree : 5.3.1
matplotlib : 3.10.1
numba : 0.61.0
numexpr : 2.10.2
odfpy : None
openpyxl : 3.1.5
psycopg2 : 2.9.10
pymysql : 1.4.6
pyarrow : 19.0.1
pyreadstat : 1.2.8
pytest : 8.3.5
python-calamine : None
pytz : 2025.1
pyxlsb : 1.0.10
s3fs : 2025.2.0
scipy : 1.15.2
sqlalchemy : 2.0.38
tables : 3.10.1
tabulate : 0.9.0
xarray : 2024.9.0
xlrd : 2.0.1
xlsxwriter : 3.2.2
zstandard : 0.23.0
tzdata : 2025.1
qtpy : None
pyqt5 : None