I've just read this here:
http://amarok.kde.org/en/Insider/Issue_15

in particular...

Collection Browser caching bug has been found and squashed flat. Restarting Amarok should no longer be necessary to get some changes in your collection to be properly displayed.

My question is now, could that be related to the same problem I've encountered in 2.5.0?
I'll try to describe:

For reasons of simplicity, let's assume I got a /media/sdb5 partition with a folder COLLECTION.
There are two subfolders in this folder, called AUDIO and NON-AUDIO.
If I now:
- select COLLECTION (in Settings->Configure Amarok->Collection)
- start full rescan ( = non-incremental)

everything still works OK.
But after a short coffee break---scanner still running!---I realize that I should NOT scan NON-AUDIO folder.
So I would go and

- stop current scan
- uncheck NON-AUDIO
- restart full rescan

And now it comes: Amarok collection scanner will STILL output debug messages about scanning NON-AUDIO folder, even though it had just been unchecked, denoting that we do no longer want that folder to be scanned.

However, after quitting Amarok entirely, then restarting, everything works as expected, and the deselected folder finally gets disregarded; though not until Amarok has been fully restarted.
So is that the bug referred by the quoted excerpt from the Insider blog post?

You do realize that this Insider issue is 2 years old, do you? What is in there about the Amarok database is no longer relevant as it has changed a lot since.
Also why do you talk about the Amarok 2.6 changelog when you actually refer to the insider issue?

You've answered the question all by yourself.
I thought the "Issue 15" was something recent and I have to admit I misread September 17 2010 as September 17 2012. My bad.