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
Related issues
Comments
I have tried on the snapshot, in a superflat world
EDIT: I have tried without full screen and the anvil doesn't duplicates. Only works with fullscreen mode activated. I'm sure that a little lag while i'm taking the screenshot creates the second anvil

Yes I agree. To me it does look like the lag caused by the screenshot gives time for the anvil to fall and another to be placed.
It's not a duplication, you're actually placing 2 anvils. Duplicate of MC-1887.
Tried it a bunch of times with no success.