I very highly doubt that the "garbage" files could get installed to the Wii U. Every file on NUS will be digitally signed by RSA and SHA-1, meaning that the chances of a garbage file being properly signed are approximately 1461501637330902918203684832716300000000000000000 to 1. It should download the garbage, then check the integrity, then delete the files if the signature is invalid and install them if it is valid. For there to be a decent chance of the signature actually being valid, there would need to be around 1.46 * 10^48 Wii Us in use. Now, if Nintendo had sold that many Wii Us, then they would be richer than Microsoft! So, granted it could happen, but the chances are higher that you will get killed in a car accident today! (If you don't believe this, then think about these facts: There are about 100 people killed in car accidents each day. If the odds were really as high as the random file signature being valid, then there should be 1.46 * 10^48 people on the earth, and only one should die in a car accident.)