Upgrades failures may happen simply because a candidate was not ready. This can happen in a seniority based system particularly at times when hiring is busy and employers need to reach down farther into the depths of the pool.
Unscheduled ops are far different than those used to domestic or flag rules. In the non-skeds you are pretty much on your own and upgrade failures happen in the sim or during ioe/line checks due to the sometimes overwhelming nature of the beast for those who are not ready.