Unfortunately, the Finder in Mac OS X contains several bugs when it comes to custom desktop icons. The Finder has a tendency to cache icons instead of updating them as it is supposed to, so when you try and set custom icons over other icons, you may not see the change immediately.
Logging out and back in (or restarting) usually takes care of this problem, but not always. We’ve reported these bugs in the Finder to Apple and await a fix in a future version of the Mac OS.
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