I'm not sure, if adding $0$ counts as an 18th camel:  

Sometimes adding $0$ expressed as the difference of two equal values or variables simplifies things; a very basic application is squaring numbers by utilizing the 3rd binomial identity $(a+b)(a-b) = a^2-b^2$ which used in the form $a^2=(a+b)(a-b)+b^2$. 
In order to calculate e.g. $35^2$ one would calculate $30\cdot40+25$.  

I have also seen proofs that utilize the trick of adding $0$ in the form $x-x$, but unfortunately I can't remember what it was.