Is facing the Qibla an obligation that is waived under certain circumstances? If so, what are they?

Facing the Qibla is a condition for the validity of prayer, based on the Almighty's saying: "So turn your face toward Al-Masjid Al-Haram. And wherever you are, turn your faces toward it." Exceptions to this include the prayer of fear (Salat al-Khawf), the crucified, the drowning, those unable to face the Qibla, and the permissible supererogatory (nafl) prayer during travel. In these cases, these conditions are waived due to an excuse or for alleviation in the voluntary prayer, as the Almighty said: "And to Allah belongs the East and the West. So wherever you [might] turn, there is the Face of Allah. Indeed, Allah is all-Encompassing and Knowing."