Quenching Risk
In normal operation the highest risk of a quench is when the current probe is inserted in order to change the field, or when liquid helium is being fed to the cryostat. Both these operations should be carried out be authorised personnel only.
Further reading on this topic:
Books: Q&A in MRI p58, MHRA Guidelines p89-90, 94, MRI From Picture to Proton p169