I noticed the following behaviour in google.
After a url structure change about 2-3 months ago my whole testsite went supplemental. I used 404 on the old pages, no room for 301.
I thought o heck :p. Anyway about a week ago i decided to use the google automated url removal tool http://services.google.com/urlconsole/controller?cmd=reload&lastcmd=login
The thing i noticed was: those pages I removed seemed to return from the supplemental index. Not all but most, and already after a few days.
My idea why this happened is: google still remembers the page behind the 404 and see's the new page as a duplicate. Does anybody have any thoughts on this subject?