User blog comment:Master Moridin/Hallucination + Inconsistency = Confused Mori?/@comment-226878-20151214045405/@comment-226878-20151215151432

The part where Yang walks away from Merc up until she shoots his leg. In reality, Yang was standing near him the whole time.

Another possibility is that the video didn't show us, the out-of-universe audience, all that went on. Maybe Merc was supposed to be shown walking up to Yang (as the in-universe audience would have seen "to congratulate her") when she suddenly turned and attacked him.

If you'll notice on the video it jumps from him on his knees to him suddenly standing next to her.