It's not particularly uncommon. Check out the mapper list at http://tuxnes.sourceforge.net/nesmapper.txt. All the 0k chr entries are carts that have chr ram. Just for funsies, I looked up one I had, and cracked it open so we could take a look at it. You can see on this Cobra Triangle cart the MB8464A 8k sram chip wired up to the chr lines. https://drive.google.com/file/d/0B2I5cGIsgvHbN1NFdUdLNTluNHM... It's also kind of cool to see the 74 series chips at work; the 74161 counter being used to select the prg rom bank, and the 7402 used to disable the prg rom during writes.