MRIs
Far too many movies and TV shows use the magnet to cover for their lazy writing by treating it like something that can be turned on and off like a light.
The magnet in an MRI is one of the coolest things in medicine, and writers get it wrong all the time. In the vast majority of cases, it's always on.
In simple terms, an electromagnet works by running a current in a circle and creating a magnetic field. In an MRI, the current is flowing in what is essentially a closed loop of wire. However, in this case the wire is cooled with liquid helium so it becomes a superconductor.
They induce a current in the wire which creates the magnetic field ("ramp up" the magnet). Because it is superconducting, the current doesn't stop. Once it's ramped up, it no longer requires any external power. As long as the current is flowing the magnetic field remains.
There are only two ways to "turn off" the magnet.
One way is to "ramp down". Essentially the opposite process that is used to get it running in the first place. That's what they do if they need to stop it for service.
The other way is to quench the magnet. You hit the emergency stop and vent off the liquid helium. Without the helium, the wire warms and resists the current and the flow stops.
Quenching a magnet is a magnificently dramatic process. Someone hits the panic button, and there is a loud roar as the helium escapes. Clouds of condensation form around the exterior of the building as the cold gas escapes. In the event some construction crew screwed up and accidentally sealed the vents, there could be an explosion from the rapidly expanding gas.
If writers want to use an MRI as a plot device, have an accident and require someone to quench the magnet to save a life. You'd have the immediate drama from the accident and the quench, and then you'd have the long term drama of the hospital trying to figure out where the money to fix the MRI would come from.
