I found this glitch or bug quite a while ago but I didn't get around to publishing it a few months later.
This isn't fake, you can finish the construction of a building for 0 gems, however it is not a hack/game breaking glitch as the bots seem to suggest.
It is most likely not as you would have assumed it to be, it is a simple bug, not game breaking.
ENJOY : )
Thanks to supercell and shout out to film cinematic for the help:
Pixler was used
Thanks To Supercell, This video isn't intended to be offensive or 'hostile' to Supercell
This video isn't intended to degrade supercell