Metallic fragments in the eye are very dangerous because that could cause eye damage or blindness by moving the metal within the eye. Your eyes do not form scar tissue as the rest of your body does. A fragment of metal in your eye that has been there for 25 years is just as dangerous today as it was then - there is no scar tissue to hold it in place. People with pacemakers cannot be scanned or even go near the scanner because the magnet can cause the pacemaker to malfunction. Aneurysm clips in the brain can be very dangerous as the magnet can move them, causing them to tear the very artery they placed on to repair.
Some dental implants are magnetic.
Most orthopedic implants, even though they may be ferromagnetic are fine because they are firmly embedded in bone. Even metal staples in most parts of the body are fine. Each time we encounter a patient with an implant or metallic object inside their body, we investigate thouroughly to make sure it is safe to scan them. Some patients are turned away because it is too dangerous. When this happens, there is usually an alternative method of imaging that can help them.