For me, it was a Kurdish soldier on the ground getting shot in the head with a AK from point blank. There were no bits and fragments flying around, his skull just broke apart and his head flattened uncannily like a squished rubber toy and his eyes popped out.

In another video, a ISIS (?) soldier gets shot in the head, and falls to the ground. We see him struggling to get up while a viscous red liquid pours out the side of his head. Just pours out. No pink mist or giblets. He didn't die instantly.

This broke the preconceptions I had from movies and videogames. I consider it an important milestone.