Using the standard copy/paste procedure to paste an icon from one Info panel to another, you'll sometimes find that the paste operation fails, and you're left with a blank / transparent icon on the target, rather than the icon you intended to use. In my experience, this usually happens when replacing hard drive icons.
Whether this is a bug in individual icons or in Apple's icon rendering system is unclear, but the fix is simple: Re-open the Info panel on the target, select the (now blank) icon well, and press Delete. The default system icon will be restored. This doesn't get you the icon you wanted, but it's better than an invisible / missing icon.
[Editor's note: I believe I've also read that if you logout and login, you may then see the 'invisible' icon you just pasted. Unfortunately, I don't recall where I read this and can't really test it until this evening...]

