Several things can affect the safety of a flight: a disruptive passenger, bad weather conditions, an illicit interference, for instance. Sometimes, the hazard comes from something that can be a symptom of a much more serious problem, such as a leak, a false indication in one of the instruments or fumes.
Passengers and crew members of a US Airways flight which was going from Charlotte, North Carolina, to Jamaica were in need of medical assistance after inhaling fumes inside the aircraft while it was still on the ground. They were all discharged shortly afterwards, and the source of the smell was not found, as you can see here, here, here and here.
What would you do if you started smelling fumes during the flight? What can this signal? Depending on where the fumes are coming from and their smell, they might be the sign of a faliure: what systems can present fumes as a symptom of malfunction?
Are you able to discuss this situation with ease in English? If not, you may not have reached level 4 yet…