Actual icons are different than the images we paste over them; the icon fileswith the extension .icnsare actually a set of images, not a single image. For system icons, the images are actually a part of the OS, and the OS will only allow them to be used with files with certain names, so, for example, if you rename your Documents folder to My Documents the system will change the folders icon to a blank folder. This is really done to make things easier for youor at least less confusing for the system itselfso that youll know which are the official folders or files.
To get around it:
Lets say that you want to use the systems Music folder icon for your Song Lyrics folder;
1.)*******Select the Music folder and then Get Info.
2.)*******Select the folders icon in the information window and Copy the icon.
3.)*******Close the infomation window and open the Preview app. In Preview, select File > New from Clipboard and you will see the images that make up the .icns file.
4.)*******Select the top image from the column and Copy it. Close and do not Save.
5.)*******Select (or create) the folder that you want to paste the image onto and then Get Info.
6.)*******Select the folders icon in the information window and Paste on your new icon icon.
Im not recommending that you do this, but, at least as far as the system goes, it wont do any harm.