Nerve and Muscle Pressure
When an implant is placed on top of muscle, nerves, or blood vessels, it has the potential to interfere with their function. In one instance, a person had a pair of star-shaped implants inserted half way up the inside of each forearm. Shortly afterwards, each star began sinking into the tissue below and can now barely be felt although they are visible due to the regular fluid retention and bruising. In addition this person is suffering from radiating pain and cold chills in the area (a condition observed frighteningly regularly). Naturally, this person is now seeking medical assistance in their removal.
Usually these problems become apparent shortly after the procedure is done, but when the complication is nerve-related, it's very possible for symptoms to be apparent to the client during the procedure itself as the tools stimulate the nerves — minimize your risk by being sure to tell your practitioner if you experience anything like that so they can assess the risk, and either abort or modify the procedure for your safety. You should also strongly consider minimizing this risk by avoiding parts of the body that cannot be safely interfered with in this way (including the vast majority of "fleshy" parts of your body, as well as locations where you can clearly see or feel things that shouldn't be interfered with such as tendons).
It should also be noted that these problems are not necessarily reversible so it is important to stop them immediately.