[stephen_g]

There was a disaster where the pressurisation switch was left in the wrong position by a maintenance crew [1], and the pilots didn't realise because they thought the alarm was for something else, and by the time they realised something was wrong they were starting to suffer from hypoxia and were no longer able to react properly. Additional warning indicator lights were added to the cockpit to make the situation much more obvious.

Now pilots would hear the warning, scan the indicators and see the indicator, and are trained to put on their oxygen masks immediately. The cabin masks would deploy automatically. Then they follow the checklist which is to descend to a safe level if the pressure is uncontrollable (which it wouldn't be if it's just switched off).

It's very unlikely the pilot would turn off cabin pressurisation, but it would be pretty gentle and warning alarms and indicators would start pretty quickly (including on EICAS on newer planes like the 777). The pilots could easily fix it well before their or the passengers' oxygen ran out.

1. https://en.wikipedia.org/wiki/Helios_Airways_Flight_522