The critical difference: final sitting performed or omitted

Prostrations of Forgetfulness: Causes, Method, Missed Sittings and Doubts

The key legal difference is whether the obligatory final sitting occurred:

• Omitted it, then rose for a fifth: return before its prostration; after its prostration, the fard is lost.

• Completed the final sitting, then rose by mistake: the essential sitting was already fulfilled, changing the status of the extra rakah.