I posted way back on the front page of this topic, right after Snow Leopard was released, with this problem. The problem occurred repeatedly, all the time, when i had 10.6.0. Then, a little while later, i updated to 10.6.1 (or possibly 10.6.2 — i can't remember), and it seemed completely resolved! I haven't had problems with it for months, it was great.
Then all of a sudden it came back the other day. :,(
The following things happened:
1. I installed the latest iTunes.
2. I installed Safari 5.
3. I installed OS X 10.6.4.
4. I installed Adobe Photoshop CS5.
(As you can see i've been putting off my updates for some time....)
After this, the problem came back with a vengeance. I am now seeing this repeatedly again, in Quicksilver, in the Dock, and in the Finder.
Things that did NOT happen:
1. I did not mess with anything in CandyBar. I haven't even used CandyBar for almost a year.
2. I did not add or remove anything in my Dock EXCEPT for taking out the old Photoshop icon and putting in the new one.
3. I do not have any custom icons directly in my Dock.
4. I do have a folder (my Downloads directory) which CONTAINS other files that have custom icons — but i've had this in the Dock since the very beginning.
Oh Apple, why won't you fix this faster :,(