So first of all where/when do you get this error cod?
If it's when device connected to iTunes then here you can find some help: http://support.apple.com/kb/TS3694
And as the art states error 1015:
Error 1015: This error is typically caused by attempts to downgrade the iPhone, iPad, or iPod touch's software. This can occur when you attempt to restore using an older .ipsw file. Downgrading to a previous version is not supported. To resolve this issue, attempt to restore with the latest iPhone, iPad, or iPod touch software available from Apple. This error can also occur when an unauthorized modification of the iOS has occurred and you are now trying to restore to an authorized, default state.
Hope it helps,