During custom firmware restoration or iPhone OS downgrade, you may encounter various kinds of iTunes errors. This guide shows how to fix Error 1015. If you are on ios4.3.4/4.3.5 then this video will notwork for you and your iPhone may just boot back into recovery or it will get a 60000ms error. Skip to the second video if you are running on 4.3.4/4.3.5
You can find the links to all the downloads required here
http://ihelpcorner.com/how-to-fix-error-1015-when-downgrading-iphone-3gs-to-4...
IOS 4.3.4/4.3.5
Here is a video that will help with 4.3.4/4.3.5 IOS
Download Links
Redsn0w 0.9.8b4::http://blog.iphone-dev.org/redsn0w-iOS5
iPhone 3GS 4.3.4 firmware file: http://www.iclarified.com/entry/index.php?enid=750
