The 737 pressurization system in manual is very easy to cock-up if you are not careful. What happened to Helios nearly happened to Ryanair and Westjet but for slightly different reasons.
It was a design flaw in the 737 that the Take-off warning Config horn sounded the same as the Cabin Alt warning horn. Pilots were more used to associating it with take-off as that is the warning they would most often hear (usually the speedbrakes not set properly). In the report it was stated that the engineers asked the crew to check whether the pressurisation was set to AUTO but the crew did not respond to the instruction. The crew also failed to properly set the panel pre-departure and after take-off. The Captain and F/O were in their 50's so the early onset of hypoxia would not have been unusual and by that stage they were fixated on the warning horn and an avionics cooling warning.
One of the issues often forgotten about is the hardened cockpit door installed as a result of 9/11. The CM could not just enter the cockpit and query the masks having deployed as he probably would have done pre 9/11. In the Ryanair case it was only the CMs persistence on the interphone that alerted the crew to the lack of pressurisation.