Secret Coin

The Secret Coin can be found in the Shop area. When in the shop, stand on top of the red hesh platform. Then, jump through the gap on the top-right and move into the skin selection box. Keep on moving left and collect the Secret Coin. On collection, you will get the achievement Secret Coin. It does not give any gold (apart from the gold you get when you collect a coin). The achievements description says "Collect the secret coin".