Steps:
1. Join online server.
2. Start placing blueprint of concrete.
3. While holding left click and placing, start running in a direction.
4. Right click (still holding left click) while your mouse is over grass or a blueprint of concrete. If you right click over placed concrete it is fine.
5. Desync.
I'm not sure if it is specific to placing concrete, but that is usually what I am doing when I hit this. I've tried on other servers as well and it happens there too. I don't know why I accidentally right click all the time

Version: 0.14.4
Client: Windows 10 x64
Server: Ubuntu 16.04.1