These are the so called "low probability yet high consequence incidents".
Proper risk assessment procedures must be used to recognize such events. Even if it is decided not to implement any safety measures against such events at the current time, i.e. considering their remote possibility, it is still important to evaluate and document such possibilities together with associated assumptions made during the assessments. Sometimes, new data can emerge during later phases (by studies, new statistics, or by new cases) that might suddenly increase the likelihood of such events. Any previous study can then come in handy to readily reassesses such events and then proceed to design safety barriers against them. Also such documentation with relevant assessments and assumptions can be really useful for future safety design engineers to reevaluate when the plant (or the industrial environment) undergoes certain changes during its lifetime. Experience shows that some previously neglected scenarios can suddenly become highly likely when certain changes in the plant environment or operating conditions happen over the time.
