What is the ruling on someone who did something they thought was permissible, but it later became clear that it was forbidden, or someone who did something they doubted was forbidden, and it later became clear that it was forbidden? And do they incur sin in both cases?

The fundamental principle is that it is not permissible for a responsible individual (mukallaf) to undertake an action until they know its legal ruling (hukm al-shar') in Islam, even if this requires asking about it. This is based on the Prophet's (peace be upon him) saying: "Seeking knowledge is an obligation upon every Muslim." Therefore, if someone performs an action believing it to be permissible, and then it becomes clear to them that it is unlawful, they bear sin due to their negligence in knowing the ruling, not due to intentionally violating the law or committing disobedience. This is because they are "excused for ignorance, but not excused for their failure to seek the truth." Furthermore, whoever embarks on an action while doubting its unlawfulness is even more deserving of sin, due to their failure to ask and their unwillingness to refrain from that which they doubt.