Just came again across a feature of Oracle Application Express (APEX) which is worth mentioning it.
Have you ever had the problem, that the page processing flow of Oracle APEX was different then what you thought it should be? And it took you some time to figure out how Oracle APEX is processing the page?
Then maybe you should check out the “Event” view on the page definition page. It shows you a brief overview how the page is processed. It even shows you the page 0 stuff.