[G3 Q&A, Help & Troubleshooting] GrayNullFox:
I'm on the same boat. I don't even know how it happened since I wasn't even trying to flash a ROM. One person told me to look at tablets hard brick but I've had no luck yet. Phone was stuck in a recovery bootloop so I tried to wipe/restore backup which didn't work. Popped the battery, went to download mode, started flashing TOT, got an error (yes I had the right files), popped battery again hoping it was a bad connection but nope. I got it the other week :/ as a replacement because my G2 had hardware issues. There's gotta be a way somewhere. In the mean time, I had my gf call my service provider claiming she got a popup about an upgrade, clicked yes and came back to a black screen. They said to bring it in because it's within 14 days of the refurbished. I'm going to take it in but I'm hoping there's a way to get the phone to not load anything at all (led or the boot error during LG), or do sales reps not know enough and/or don't care enough?

I got it working!!! Somehow I got TWRP'd to boot then restored my backup of 4.4.2 stock. AT&T connecting/dropping and Baseband: "Unknown" before it randomly rebooted into download mode which I'm at now. I tried flashing TOT through COM41 but it wasn't successful. Not sure what to do now.

I had some paragraph typed and it must've not gone through. I got it back to the home screen and AT&T logo once "firmware upgrade" was an option again. I still don't know how I switched it from Qualcomm to LGE Mobile, maybe some files I deleted thinking an app backdoor'd it and injected files. After a fun and exciting 12+ hours straight with errors, bootloops, reading forums, videos, downloading/un-install. I managed a way that worked and booted back into a fresh device.

Hopefully this method will work for you, and who ever else has the same problem.

Here's part of the log. Not sure if hopeful or not.

[ 9:46:27] == PROPERTY INFO

[ 9:46:27] 1. download cable = USER

[ 9:46:27] 2. battery level = 77

[ 9:46:27] 3. download type =

[ 9:46:27] 4. download speed = 0

[ 9:46:27] 5. usb version = UHS

[ 9:46:27] 6. hardware revision = rev_10

[ 9:46:27] 7. download sw version =

[ 9:46:27] 8. device sw version = D85010d

[ 9:46:27] 9. secure device = S

[ 9:46:27] 10. laf sw version = 1.0

[ 9:46:27] 11. device factory version = LGD850AT-01-V10d-310-410-JUN-19-2014+0

[ 9:46:27] 12. device factory out version = LGD850AT-00-V10d-ATT-US-JUN-19-2014+0

[ 9:46:27] 13. pid = HD58S140925001695

[ 9:46:27] 14. imei = XXXXXXXXXXXXXXXXXXXX

[ 9:46:27] 15. model name = LG-D850

[ 9:46:27] 16. device build type = U

[ 9:46:27] 17. chipset platform = msm8974

[ 9:46:27] 18. target_operator = ATT

[ 9:46:27] 19. target_country = US

[ 9:46:27] 20. ap_factory_reset_status = 2

[ 9:46:27] 21. cp_factory_reset_status = 0

[ 9:46:27] 22. isDownloadNotFinish = 0

[ 9:46:27] 23. qem = 0

[ 9:46:27] 24. cupss swfv = FFFFFFFFFFF-FFFFFFFFFFF-FFFFFFFFFFF-FFFFFFFFFFF-F

[ 9:46:27] 25. is one binary dual plan = 0

[ 9:46:27] 26. memroy size = 61071360

[ 9:46:27] 27. memory_id = 032GE4

[ 9:46:27] 28. bootloader_ver = MiniOS 3.0

[ 9:46:27] LAF : Bin_User_Mode

[ 9:46:27] isValidateSecureImage Device is a qfused.

[ 9:46:27] found sbl1 Partition for secure image check

[ 9:46:27] Secure Image

[ 9:46:27] found aboot Partition for secure image check

[ 9:46:27] Secure Image

[ 9:46:27] found rpm Partition for secure image check

[ 9:46:27] Secure Image

[ 9:46:27] found tz Partition for secure image check

[ 9:46:27] Secure Image

[ 9:46:27] Secure Image Check Success

[ 9:46:27] * Set AP factory_reset= 0

[ 9:46:27] * Set qem_reset= 1