If you're planning on taking Super Smash Bros. for 3DS online, you'll need to download a patch first.
The Nintendo Support site explains anyone looking to play against the world in Super Smash Bros. for 3DS, will need to make sure they've got the latest patch installed, otherwise it won't work.
"As of October 2, 2014, new update data for Super Smash Bros. has been made available for download in the Nintendo eShop," it reads. "Users will be required to download this update prior to using the software's online multiplayer features. Because of this, we strongly recommend that all Super Smash Bros. players download and install this update data."
Those planning on picking up Smash Bros. today have two options when it comes to updating. The first is to head on over to the Nintendo eShop with Smash Bros. inserted or downloaded, then the home menu should show a list of available software update data. Select the correct update, and it'll start a download to the memory card.
"Alternatively, starting Nintendo eShop on a Nintendo 3DS system that is displaying Super Smash Bros. on the HOME Menu will show a list of software with update data available, from which you can select and download the update data."
Nintendo also notes that once the download is complete, check the Smash Bros. main title screen to confirm that it reads '1.0.2.'
The other option for installing the patch, and the easier to understand version, is to make sure your 3DS system is up to date with the latest firmware (go to the settings menu then 'perform system update'). If everything's as it should be, simply scan this handy QR code and follow the on-screen instructions.
