A wife leaves the house without her husband's permission: is it a sin?

According to the Shafi'i school, a wife needs her husband's permission to leave the house, except in an emergency or a Shariah-recognised need.
Permitted needs include obtaining emergency treatment, escaping danger, seeking personal religious obligation (fardu ain) knowledge that the husband does not teach, or going to court to claim her rights. Permission need not be asked every time; general permission, such as for going to work or shopping, is enough. The husband in turn may not prevent her without reasonable cause. The Prophet ﷺ also forbade husbands from preventing their wives from going to the mosque. See Rights of husband & wife.