Web toolkits sometimes to evolve to the point where it becomes hard to figure out what file is responsible for a particular part of the page, and consequently difficult to debug or modify those pages when debugging or modification is necessary. It’s a byproduct of getting the ability to nest a page in a standard frame, or being able to include resuable templates from a template library. OpenACS mitigates the difficulty by running in a “developer mode” that lets you see where the different parts of the page come from.