Is a traveler who stood for the third rak'ah in a prayer he intended to shorten obligated to return immediately, or should he complete his prayer? And what are the implications in both cases?

It is best for the worshipper to return in his prayer and complete it as two rak'ahs as he intended, and to perform the prostration of forgetfulness (sujud al-sahw) after the taslim. However, if he completes the prayer as four rak'ahs, there is no harm in it.