What is the legal consequence if a worshipper forgetfully omits a necessary act (Wajib) during prayer?

The Necessary Acts of Prayer (واجبات الصلاة) Part 1: Recitation

• The prayer is completely invalidated and must be repeated immediately.
• The omission is automatically excused and no action is required.
• The prayer can be rectified by performing the prostrations of forgetfulness (Sujud al-Sahw) at the end.
• The worshipper must restart the ablution.