Nintendo's 3DS isn't just a 3D portable games console, it's also apparently a style judge. According to the handheld's user guide, players with beards, bangs or even glasses could end up confusing the 3DS' facial recognition system.
Piercings and bangs can also apparently confuse matters, while the environment you play in - what's behind you, how good the lighting is - can also have a big impact. Basically, it sounds like the 3DS would prefer you to follow the same guidelines as for passport photos: clear face, plain background and, whatever you do, no smiling.
Now, we've been confused by beards before ourselves in the past, but we do worry about long-sighted gamers forced to squint at their screens because the 3DS forces them to take off their reading glasses if they want to be recognized properly. Bearded, glasses-wearing and riddled with piercings? Perhaps you should just save your money for an NGP.