A McCree player sees a sneaking Tracer in the distance and decides to investigate. And the player's intuition paid off. While the Tracer snuck up to the high ground, it was unknown what it planned.
Once our hero got to the top, they realized the Tracer was probably setting up and Pulse Bomb on his unsuspecting team below, and probably an assassination attempt on Widowmaker. You can almost feel the frustration of Tracer after spending so much time crouch-walking toward the top.
Redditor HowCouldUBMoHarkless posted the clip of the hilarious interaction between the two characters. Just goes to show, trust your gut when you play Overwatch. If you see an enemy player sneaking up a back corridor directly behind your team, go investigate. It may just save your teammates life, or even more importantly, a whole team push.
The Overwatch subreddit is full of these plays including an Ana who completely outplays an enemy Widowmaker. It's something special to behold as the Ana lands a beautiful Sleep Dart on the fleeing Widow to finish the duel off.