Reads the full error (via Soft):
An error has occurred. Hold down the Power button to turn off power, then turn it on and try again. If the problem persists, please contact your local customer support center.
The worst part of the error is it is still unclear what triggers it, and it appears that the error screen is frequent for some users.
Nintendo did respond, but it was not very revealing:
Our recommendations are that if anyone is experiencing any problems with their Nintendo 3DS console, we recommend that in the first instance they download and install the latest system update, now available online. If the problems still persist we recommend they contact their local Nintendo Customer Service centre to investigate the problem.