As an example of the sort of old design which used to be grandfathered in there's the Helios crash [1], where on a plane built in 1997 the exact same alarm horn

As an example of the sort of old design which used to be grandfathered in there's the Helios crash [1], where on a plane built in 1997 the exact same alarm horn was used for two situations (configuration error during take-off, and loss of cabin pressure at altitude). The old-school reasoning was that this wouldn't be a problem since the two situations are necessarily disjoint. However as the accident investigation board wrote: "Most pilots are not very likely to experience a cabin pressurization problem and the associated warning horn at any time during their line flying. The Board also considered the role of stress that probably further contributed to the possible confusion of the two meanings of the warning horn. In general, stress, such as that caused by the onset of a loud, distracting alarm in the cockpit, combined with the element of surprise, is known to lead to automatic reactions. Automatic reactions, in turn, are typically those that result from experience and frequency of encounter and are therefore not always appropriate. The Board considered that the flight crew may have automatically reverted to a reaction based on memory before consciously processing the source and significance of the stress factor. This would also explain why the flight crew failed to realize the improbability of their interpretation of the horn as a takeoff configuration horn and why they failed to move on to gathering information for a new, correct diagnosis of the problem at hand." That particular stable door got closed, but at the time it was cited by pilots as an example of legacy design that wouldn't have been permitted on a clean-sheet airliner. [1] https://en.wikipedia.org/wiki/Helios_Airways_Flight_522#Flight_and_crash https://en.wikipedia.org/wiki/Helios_Airways_Flight_522#Flig...