The source also discusses:
• A person joins a prayer leader after the leader has already given Salaam but still owes Sujud al-Sahw: the following remains valid if the leader performs the pending Sahw; otherwise it does not remain valid in that form.
• A person who gives Salaam intending to leave while still owing Sahw may still perform it if no incompatible act, speech or turning away has occurred.
• If a person mistakenly gives Salaam after 2 rak'ahs of a 3- or 4-rak'ah prayer thinking the prayer is complete, the remaining rak'ahs are completed and Sujud al-Sahw is performed.
• Prolonged uncertainty about the number of rak'ahs for the duration of a prayer-pillar can itself require Sujud al-Sahw.
These cases show why Sahw should be taught as a system, not merely 'two prostrations after any mistake.'