The problem is when there are already list content types based on a site content type and we need to add a new event handler to it and have all list content types also updated. Feel free to adapt or improve as desired or required. One of the faults I ran recently into is related with updating list content types event handlers when site content types event handlers are updated. Synchronous events such as "ItemAdding" or "ItemUpdating" occur before the action has taken place. For instance, you may want to scrub the data after it has been entered, or you may want to email the administrator if a field has been added - as an FYI. Content types basically define the metadata they are composed by a reusable set of columns called site columns and behaviors event handlers, workflows, etc of a certain category of content.