The risks of calf augmentation, as with any surgery, is a (low) risk of post-operative infection, bleeding and swelling.
Some risks and side effects include pain, scarring, allergic reactions, skin loss, and skin discoloration. There may also be fluid or blood accumulation. There is also a risk of the silicone implants slipping out of place or rupturing and correcting this may involve further surgery.
Your surgeon will discuss all the risks with you before the procedure.