As m_turner mentioned, catastrophic quenching is an important feature of MRI scanners. These scanners basically contain incredibly strong superconducting magnets (1.5 Tesla, for those of you who know about such things). As the safety video says,

The Magnet is Always On

and it doesn't shut off when you're not actively scanning. Thus, if you bring anything ferromagnetic into the scanning room, it's likely to go flying into the scanner. That's a minor problem if the scanner is empty and it's a small object like a paperclip; you can easily reach in and pull it off (though it'll take quite a tug). If, however, you do something even dumber--like bringing a wheelchair, oxygen bottle, or sledgehammer into the scanning chamber while a patient is lying in there--it too will go flying into the scanner at high speed aimed right at the patient's head. Now remember,

The Magnet Is Always On

and human beings aren't strong enough to yank a heavy object out of the scanner and off the patient (who is currently bleeding to death). In this case, you push the Big Red Switch located on the scanner or the operating console, which vents the coolant, wrecks the superconductivity, scrams the magnet, fills the room with vapor, and allows you to get the heavy metal object off the patient. It's a procedure about as serious as venting the warp core, in that you only do it when you're really screwed; it also has about a fifty-fifty chance of trashing the multi-million-dollar MRI scanner, so you don't want to hit it by accident (as my advisor once did before they installed the molly-guard). Although you never want to see someone quench an MRI scanner, it's still good to know that the safety feature's there.