On the sidebar of the main page, there is a section about referrals.

Gold Coins are credited after your referral finishes tutorial. To avoid abuse, we allow to get only one referral prize from every unique IP.

The first sentence is missing an article and the second sentence is just...wrong? I'm not sure how to describe the problem, but I rewrote it so it is correct.

Gold Coins are credited after your referral finishes the tutorial. To avoid abuse, we only allow one referral prize for each unique IP.