Here is what I did on my D802 to recover my phone from fastboot brick(thought might be useful specifically for d802 users). I was able to get into download mode though. If you are not able to get into download mode, follow this
1) Wipe cache,system etc except internal storage
2) Download mahdi rom
3) Download gapps
4) flashed rom but forgot to flash the patch for d802.
5) After restart, the phone went straight into fastboot mode
450 Fastboot mode started
740 fastboot; processing commands
I tried adb but did not detect the device. Also, driver was not getting installed properly. Tried many drivers like the universal driver etc etc.
What worked for me was
1) Uninstall all drivers like LG,Samsung,Nokia from Laptop
2) Restart machine
3) Remove phone from usb cable
4) Download pdanet+ from here
5) Install downloaded pdanet by selecting modal as LG, at some point it will ask you to put your phone into usb debugging mode in a popup
6) Just press volume up + power which will take you into download mode connecting the usb cable
7) The popup should automatically vanish and the drivers will get installed
8) Next, followed the steps specified exactly in this post
9) If you skip any step, you loose
10) I tried to flash stock 4.2 initially and got TYPE_WPARAM_UPGRADE_ERROR. If you get this error, its probably you have already installed 4.4 rom previously. Hence download only correct stock rom KDZ which is previously installed.