Tell me when you flashed your Mastery...
...And it indicated an embarrassing Fail moment.
Don't worry, I'll start by telling a few of my own.
Champion: 
Mastery Level : 7
It was a 3v1, with the enemy team being the 3, and me being the 1. Well, I was relatively fed, so I was feeling cocky. I went on them, and things initially were going my way. I get my Noxian Might, AND THEN... ######I dunked the wrong target, forcing it on cooldown. I then proceeded to die, and I flashed my mastery while saying in the chat...
"L7 Mastery Darius here, folks! I dunked the wrong guy, I DEFINITELY deserve this Mastery Token!"
Here's another one...
Champion: 
Mastery Level : 4
I had my ult up, and my target was running away. I figured "It's now or never!" So I launch myself at them... ######And I missed by the tiniest margin. I proceeded to die as they used their CC to lock me under the turret. Flash that Mastery!
One more for good measure...
Champion: 
Mastery Level: 6
My enemy was running, and all I had available was my Q. They had already used their
so it was a guaranteed hit, right? I fire away...
######And I hit Terrain instead.
You KNOW that's an excellent time to flash the Mastery!
Now that I've embarrassed myself, let's hear yours!