What character did your opponent for that match used and what action did take place from your opponent in that round immediately before your Mei was prevented from using her mystery?

Cause due to lag time and connection between both players and server, if he anticipated your Mei's mystery is going to be used, if he Acupuncture/Immobile/Chaos/etc her before you clicked your mystery, your screen will show the inability to use the skill in advance, even though on screen, the animation of that debuff have yet to be shown.

Oh yes, that's possible too.

Considering thread starter did not mention when exactly he attempted to activate her mystery (i.e. After Mei's standard attack but before the clone was killed towards the end of Round 2), hoping he might combo off her mystery to get her Immobile Chase in to get an advantage for himself over the opponents in Round 3.