Wilkes Joiner demonstrates how to do live debugging in Seaside:

I've created a short (3min) video showing how you can deal with a stack trace while developing a Seaside application. One thing to note is that most of this power and flexibility comes from the Smalltalk environment itself.

As Wilkes notes, this isn't a Seaside specific piece of power - you can do the same thing in any application, due to the power of the Smalltalk system. The server running this blog is typically updated by just filing in patch code rather than the more common (over in the mainstream software world) kill/load new code/restart paradigm.