Because I'm currently in a redevelopment phase here, moving code between domains and services, I can see from my Apache logs that there are many 404s being sent out as pages — temporarily — move around.
I've also got an issue to solve in that whilst most of my blogging over the years has used a permalink of site-date-title for a while I used site-title-date. I'm sure I had a good reason at the time, but I can't recall it now.
Edent has some ideas on how to deal with one's own outbound dead links though. Worth reviewing.