Alexban, good question.
the only protection you have from this situation is the 'negative pressure relief valve'. Thats the flapper door that opens inwards. This is simply a spring loaded door; once the pressure outside the aircraft is higher than inside then the door opens against (relatively) light spring pressure. Now the cabin will in theory descend at the same rate as the aircraft, however this is limited by the size of the opening of the relief valve, so there will be a "slight" negative pressure diff on the aircraft if the crew continues to descend at a high rate.
Although I haven't seen it written anywhere, if you do catch the cabin it is prudent to reduce rate of descent to less than 1000fpm for pax comfort and to avoid exposing the aircraft to negative pressure diff's (which the structure does not like very much)