[Nexus 7 Original Development] KrzychuG:
Ok, it seems that it stucked in some weird mode. I waited untill it popped up as unknown devices, kept power button pressed to power it off and then hit vol+ for APX. It's detected again, it looks like that:

C:\Nexus 7\2\nvflash-tools-win32>nvflash --setbct --create --configfile flash.cf
g --bl bootloader.ebt --bct recovery.bct --blob blob.bin
Nvflash v1.13.87205 started
Using blob v1.13.00000inatiŞ*║ęŘV
chip uid from BR is: 0x0000000000000000015d25644444140c
rcm version 0X30001
System Information:
chip name: unknown
chip id: 0x30 major: 1 minor: 3
chip sku: 0x83
chip uid: 0x0000000000000000015d25644444140c
macrovision: disabled
hdcp: enabled
jtag: disabled
sbk burned: true
dk burned: true
boot device: emmc
operating mode: 4
device config strap: 1
device config fuse: 17
sdram config strap: 0

sending file: recovery.bct
- 6128/6128 bytes sent
recovery.bct sent successfully
downloading bootloader -- load address: 0x80108000 entry point: 0x80108000
sending file: bootloader.ebt
- 2146912/2146912 bytes sent
bootloader.ebt sent successfully
waiting for bootloader to initialize
bootloader downloaded successfully
setting device: 0 3
failed executing command 11 NvError 0x120002
command failure: create failed (bad data)
bootloader status: specified device is invalid (code: 6) message: nverror:0x4 (0
x4) flags: 0

C:\Nexus 7\2\nvflash-tools-win32>nvflash --resume --download 8 boot.img --go
Nvflash v1.13.87205 started
[resume mode]
sending file: boot.img
| 5113856/5113856 bytes sent
boot.img sent successfully

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot erase misc
erasing 'misc'...
OKAY [ 0.023s]
finished. total time: 0.024s

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot flash bootloader bootloader-grouper-4.23.img
sending 'bootloader' (2100 KB)...
OKAY [ 0.257s]
writing 'bootloader'...
FAILED (remote: (BadParameter))
finished. total time: 0.393s

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot flash bootloader bootloader-grouper-3.41.img
sending 'bootloader' (2092 KB)...
OKAY [ 0.256s]
writing 'bootloader'...
FAILED (remote: (BadParameter))
finished. total time: 0.394s

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot flash bootloader bootloader-grouper-4.13.img
sending 'bootloader' (2096 KB)...
OKAY [ 0.256s]
writing 'bootloader'...
FAILED (remote: (BadParameter))
finished. total time: 0.393s

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot flash bootloader bootloader-grouper-4.23.img
sending 'bootloader' (2100 KB)...
OKAY [ 0.257s]
writing 'bootloader'...
FAILED (remote: (BadParameter))
finished. total time: 0.394s

boot.img is from "nakasi-ktu84p-factory-76acdbe9", bootloader-grouper-4.23.img is the one that says "sigmature match" with df53028033c9eccf4fe5ba7bc198ce24 MD5. Don't mind looking at file path. I'm using Windows 8.1 64 bit as main OS, and i do the APX part on Windows 7 VM but fastboot on 8.1.

I can flash recovery just fine for example, it's the bootloader that's giving me the headache. I can't flash older, like 3.41. 4.13 or 4.18 as well.

d:\Apps\Android\Nexus 7\5.0\nakasi-jwr66y-factory-1e503532\nakasi-jwr66y-factory
-1e503532\nakasi-jwr66y>fastboot flash recovery ..\..\..\..\WUG\data\openrecover
y-twrp-2.7.1.1-grouper.img
sending 'recovery' (8502 KB)...
OKAY [ 0.993s]
writing 'recovery'...
OKAY [ 0.573s]
finished. total time: 1.569s

That's all the files i backed up for nvflash:

2014-08-30 10:23 6*128 recovery.bct
2014-08-30 10:23 2*146*912 bootloader.ebt
2014-08-30 10:32 5*242*880 factory-config.img
2014-08-30 10:23 649 blob.log
2014-08-30 10:32 524*288 mda.img
2014-08-30 10:23 0 upload.response
2014-08-30 10:23 6*128 create.bct
2014-08-30 10:23 2*258*955 blob.bin
2014-08-30 10:32 1*048*576 eks.img
2014-08-30 10:31 11*010*048 bricksafe.img

The good thing is, i flashed bricksafe again and can go enter bootloader and access fastboot normally again.