BUG: Series of bools with length mismatch does not raise when used with .iloc

[arthurlw]

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

import pandas as pd
import numpy as np

s = pd.Series([1, 2, 3])

mask_series = pd.Series([True, False, True, True])
result = s[mask_series]

print(result)
# Output:
# 0    1
# 2    3
# dtype: int64

mask_array = np.array([True, False, True, True])
print(s[mask_array])
# IndexError: Boolean index has wrong length: 4 instead of 3

Issue Description

When using .iloc with a boolean Series mask whose length exceeds the target, pandas does not raise an error. This is inconsistent with numpy bool indexing, which raises an IndexError.

Expected Behavior

.iloc should raise if the boolean Series mask length doesn’t match the target Series length.

Installed Versions

INSTALLED VERSIONS

commit : 25e57c3
python : 3.12.9
python-bits : 64
OS : Darwin
OS-release : 24.0.0
Version : Darwin Kernel Version 24.0.0: Mon Aug 12 20:49:48 PDT 2024; root:xnu-11215.1.10~2/RELEASE_ARM64_T8103
machine : arm64
processor : arm
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 3.0.0.dev0+2080.g25e57c3415
numpy : 1.26.4
dateutil : 2.9.0.post0
pip : 25.0
Cython : 3.0.12
sphinx : 8.1.3
IPython : 9.0.2
adbc-driver-postgresql: None
adbc-driver-sqlite : None
bs4 : 4.13.3
blosc : None
bottleneck : 1.4.2
fastparquet : 2024.11.0
fsspec : 2025.3.0
html5lib : 1.1
hypothesis : 6.130.4
gcsfs : 2025.3.0
jinja2 : 3.1.6
lxml.etree : 5.3.1
matplotlib : 3.10.1
numba : 0.61.0
numexpr : 2.10.2
odfpy : None
openpyxl : 3.1.5
psycopg2 : 2.9.6
pymysql : 1.4.6
pyarrow : 19.0.1
pyreadstat : 1.2.8
pytest : 8.3.5
python-calamine : None
pytz : 2025.2
pyxlsb : 1.0.10
s3fs : 2025.3.0
scipy : 1.15.2
sqlalchemy : 2.0.10
tables : 3.10.2
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

[rhshadrach]

pandas will align on the index when you provide a Series. This is expected behavior. Closing.