What happened is that people who used the system very day, day in and day out, became so fast at entering the machine settings the rate of UI events exceeded the ability of the custom monitor software written for the machine to respond correctly to them.
Which is still to some extent a UI issue.
But the literal "killer" is what happened next:
1) The machine detected that it had screwed up.
2) But the UI reported this by a cryptic error message: "MALFUNCTION nn" - where the 1 = nn = 64 error codes not only weren't explanatory, but weren't even included in the manual.
3) And if the operator hit "P" (for "proceed") the machine would GO AHEAD AND OPERATE in the known-to-be-broken mode, giving the patient a fatal (high-power, not-swept-around) electrons rather than a 100x weaker flood of x-rays, with NO FURTHER INDICATION that something is still wrong (unless you count the patient sometimes screaming and running out of the room.)
If 2) and 3) aren't user interface problems, what is?