Adhering to a madhhab when one knows that other madhhabs have stronger evidence
If a person adheres to a particular madhhab, then he finds out that it is likely that another madhhab has stronger evidence, this is a serious error. It is not permissible to do that. This is included in what is mentioned in the aayah (interpretation of the meaning):
“They (Jews and Christians) took their rabbis and their monks to be their lords besides Allaah (by obeying them in things which they made lawful or unlawful according to their own desires without being ordered by Allaah), and (they also took as their Lord) Messiah, son of Maryam (Mary), while they (Jews and Christians) were commanded [in the Tawraat (Torah) and the Injeel (Gospel)] to worship none but One Ilaah (God — Allaah) Laa ilaaha illa Huwa (none has the right to be worshipped but He). Praise and glory be to Him (far above is He) from having the partners they associate (with Him)”
This implies turning away from the guidance of Allaah and the Sunnah of His Messenger (peace and blessings of Allaah be upon him).