Well, Seaside does have explicit code to support the debug link. If you get an exception during rendering, the framework generates a 500 response with a "Debug" link. Clicking the link restarts the previous request, and when it hits the exception, it opens a debugger instead of generating the 500 response. If you fix the error and continue, the page is sent to the browser as usual.

It's not a lot of code, but it's pretty clever.