Sometimes, placing an anvil while you are saving screenshot, duplicates it

What I expected to happen was...:
The anvil wouldn't duplicate
What actually happened was...:
Placing an anvil while you are saving screenshot, duplicates it
Steps to Reproduce:
1. While you are putting an anvil with a free block behind, press F2
Video:
http://youtu.be/TDanHLIWeE0
For me, only works with fullscreen mode activated