FAILED (remote: 12 signature verify fail) during fastboot downgrading.

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http://forum.xda-developers.com – I'm (or I was) about to root my desire Z. Using strawmetal pdf as instructions first two steps went smooth. Everything as written with no errors. Problems start with downgrading to stock froyo. Frist I've tried manual downgrade, but the phone did nothing after finding PC10IMG.zip (well it checked it and werify, but then just went back to bootloader with no message). So I reboot the phone and it loaded normaly to my system. Then I tried second method Fastboot downgrade, but when I enter into a black screen with white/silver htc logo and type: Code: fastboot flash zip StockRom.zip it gi (HowTos)