Quench Damage
Spontaneous quenching may cause severe and irreparable damage to the superconducting coils and so a decision to quench should only be made in extreme cases. It is a misconception that there can only be total quenches. Some magnets only partially quench and so the field is not completely removed. In such cases care should still be taken when handling ferromagnetic objects near the magnet.
Further reading on this topic:
Books: Q&A in MRI p58, MHRA Guidelines p49