It is now accepted that MR imaging does not present any biological risk for humans, even in cases of repeated exposure. However, several of the MR components (magnetic field, gradients, RF pulses, electrodes...) may cause some inconveniences to patients, most of them being reversible. However, severe accidents have been reported. Even though screening of patients for MR imaging eligibility is performed to identify patients with contra-indications to MRI, the lack of vigilance or the ignorance of certain basic safety requirements could lead to serious adverse effects, including death. The goal of this article is to review the various accidents reported with MRI, to explain their mechanism, and to describe means of prevention.