Has it happened before?
The magnetic force exerted on an object increases exponentially as it nears the magnet. Imagine standing 15 feet (4.6 meters) away from the magnet with a large pipe wrench in your hand. You might feel a slight pull. Take a couple of steps closer and that pull is much stronger. When you get to within 3 feet (1 meter) of the magnet, it's likely the wrench will be pulled from your grasp. The more mass an object has, the more dangerous it can be -- the force with which it's attracted to the magnet is much stronger. Mop buckets, vacuum cleaners, IV poles, oxygen tanks, patient stretchers, heart monitors and countless other objects have all been pulled into the magnetic fields of MRI machines. Smaller objects can usually be pulled free of the magnet by hand. Large ones may have to be pulled away with a winch, or the magnetic field may even have to be shut down.
Prior to a patient or support staff member being allowed into the scan room, he or she is thoroughly screened for metal objects -- and not just external objects. Often, patients have implants inside them that make it very dangerous for them to be in the presence of a strong magnetic field. Metallic fragments in the eye are very dangerous because moving those fragments could cause eye damage or blindness. People with pacemakers can't 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 were placed on to repair.
As you can see, MRI magnetic fields are incredibly strong. If a piece of metal were missed during your screening, it could cause a problem. Jewelry flying from your body and into the MRI machine is entirely possible.