It would appear that plans are afoot to change the packaging of 3DS games, presumably to help them stand out from regular DS titles.
Both GAME and Asda have placed red stickers on their 3DS ranges, covering up the usual white 3DS logo on the right hand side and adding an extra 3DS logo on the bottom of the packaging. We wouldn’t be surprised if these red borders became standard for upcoming releases.
We haven’t heard stories of people buying 3DS games thinking they’ll play on a DS but we wouldn’t put it past some. Heck, we still recall the stories of clueless gamers trying to take PSP UMDs out of their plastic casing.