[Sprint HTC One (M8)] EarlyMon:
Ok, I've searched, maybe I've missed something - anyway - this is far from my first rodeo with firmware.

Wife has stock One M8, Sprint of course. After receiving the M update, going to recovery (to clear cache) just nets the dreaded red triangle with exclamation mark.

So, I flashed the signed zip.

mv signed_6.20.651.3_firmware.zip firmware.zip
fastboot oem rebootRUU
fastboot flash zip firmware.zip

Results:

target reported max download size of 1826418688 bytes
sending 'zip' (54984 KB)...
OKAY [ 2.657s]
writing 'zip'...
(bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) checking main version...
(bootloader) checking hboot version...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) total_image_number=13

(bootloader) start image[adsp] unzipping & flushing...
(bootloader) [RUU]UZ,adsp,0
(bootloader) [RUU]UZ,adsp,10
(bootloader) [RUU]UZ,adsp,21
(bootloader) [RUU]UZ,adsp,33
(bootloader) [RUU]UZ,adsp,43
(bootloader) [RUU]UZ,adsp,54
(bootloader) [RUU]UZ,adsp,65
(bootloader) [RUU]UZ,adsp,76
(bootloader) [RUU]UZ,adsp,88
(bootloader) [RUU]UZ,adsp,98
(bootloader) [RUU]UZ,adsp,100
(bootloader) [RUU]WP,adsp,0
(bootloader) [RUU]WP,adsp,100
(bootloader) ...... Successful
(bootloader) current_image_number=0

(bootloader) start image[boot] unzipping & flushing...
(bootloader) [RUU]UZ,boot,0
(bootloader) [RUU]UZ,boot,8
(bootloader) [RUU]UZ,boot,16
(bootloader) [RUU]UZ,boot,24
(bootloader) [RUU]UZ,boot,32
(bootloader) [RUU]UZ,boot,40
(bootloader) [RUU]UZ,boot,47
(bootloader) [RUU]UZ,boot,52
(bootloader) [RUU]UZ,boot,60
(bootloader) [RUU]UZ,boot,69
(bootloader) [RUU]UZ,boot,78
(bootloader) [RUU]UZ,boot,84
(bootloader) [RUU]UZ,boot,92
(bootloader) [RUU]UZ,boot,97
(bootloader) [RUU]UZ,boot,100
(bootloader) [RUU]WP,boot,0
(bootloader) [RUU]WP,boot,99
(bootloader) [RUU]WP,boot,100
(bootloader) ...... Successful
(bootloader) current_image_number=1

(bootloader) start image[hboot] unzipping & flushing...
(bootloader) [RUU]UZ,hboot,0
(bootloader) [RUU]UZ,hboot,50
(bootloader) [RUU]UZ,hboot,100
(bootloader) signature checking...
(bootloader) [RUU]WP,hboot,0
(bootloader) [RUU]WP,hboot,99
(bootloader) [RUU]WP,hboot,100
(bootloader) ...... Successful
(bootloader) current_image_number=2

(bootloader) start image[recovery] unzipping & flushing...
(bootloader) [RUU]UZ,recovery,0
(bootloader) [RUU]UZ,recovery,9
(bootloader) [RUU]UZ,recovery,15
(bootloader) [RUU]UZ,recovery,20
(bootloader) [RUU]UZ,recovery,29
(bootloader) [RUU]UZ,recovery,34
(bootloader) [RUU]UZ,recovery,39
(bootloader) [RUU]UZ,recovery,45
(bootloader) [RUU]UZ,recovery,56
(bootloader) [RUU]UZ,recovery,63
(bootloader) [RUU]UZ,recovery,68
(bootloader) [RUU]UZ,recovery,73
(bootloader) [RUU]UZ,recovery,78
(bootloader) [RUU]UZ,recovery,88
(bootloader) [RUU]UZ,recovery,93
(bootloader) [RUU]UZ,recovery,100
(bootloader) [RUU]WP,recovery,0
(bootloader) [RUU]WP,recovery,78
(bootloader) [RUU]WP,recovery,99
(bootloader) [RUU]WP,recovery,100
(bootloader) ...... Successful
(bootloader) current_image_number=3

(bootloader) start image[rpm] unzipping & flushing...
(bootloader) [RUU]UZ,rpm,0
(bootloader) [RUU]UZ,rpm,100
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=4

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) current_image_number=5

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=6

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) current_image_number=7

(bootloader) start image[sp1] unzipping & flushing...
(bootloader) [RUU]UZ,sp1,0
(bootloader) [RUU]UZ,sp1,99
(bootloader) [RUU]UZ,sp1,100
(bootloader) [RUU]WP,sp1,0
(bootloader) [RUU]WP,sp1,100
(bootloader) ...... Successful
(bootloader) current_image_number=8

(bootloader) start image[tz] unzipping & flushing...
(bootloader) [RUU]UZ,tz,0
(bootloader) [RUU]UZ,tz,100
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=9

(bootloader) start image[wcnss] unzipping & flushing...
(bootloader) [RUU]UZ,wcnss,0
(bootloader) [RUU]UZ,wcnss,22
(bootloader) [RUU]UZ,wcnss,46
(bootloader) [RUU]UZ,wcnss,68
(bootloader) [RUU]UZ,wcnss,98
(bootloader) [RUU]UZ,wcnss,100
(bootloader) [RUU]WP,wcnss,0
(bootloader) [RUU]WP,wcnss,100
(bootloader) ...... Successful
(bootloader) current_image_number=10

(bootloader) start image[radio] unzipping & flushing...
(bootloader) [RUU]UZ,radio,0
(bootloader) [RUU]UZ,radio,9
(bootloader) [RUU]UZ,radio,18
(bootloader) [RUU]UZ,radio,27
(bootloader) [RUU]UZ,radio,32
(bootloader) [RUU]UZ,radio,41
(bootloader) [RUU]UZ,radio,50
(bootloader) [RUU]UZ,radio,59
(bootloader) [RUU]UZ,radio,68
(bootloader) [RUU]UZ,radio,73
(bootloader) [RUU]UZ,radio,78
(bootloader) [RUU]UZ,radio,86
(bootloader) [RUU]UZ,radio,91
(bootloader) [RUU]UZ,radio,100
(bootloader) trying to rename MBA
(bootloader) [RUU]WP,radio,0
(bootloader) [RUU]WP,radio,36
(bootloader) [RUU]WP,radio,73
(bootloader) [RUU]WP,radio,100
(bootloader) ...... Successful
(bootloader) current_image_number=11

(bootloader) start image[rcdata] unzipping & flushing...
(bootloader) [RUU]UZ,rcdata,0
(bootloader) [RUU]UZ,rcdata,100
(bootloader) [RUU]WP,rcdata,0
(bootloader) [RUU]WP,rcdata,100
(bootloader) ...... Successful
(bootloader) current_image_number=12

OKAY [ 23.565s]
finished. total time: 26.222s

And I still can't get into stock recovery, just the dreaded red triangle happens. If you let it sit long enough, it boots normally into the system (just saying fwiw, because I already know how to get out of that by hand).

Seems to be the only issue I could see was with image[sbl1]. Recovery seemed to flash fine.

What am I missing? Trying it a second time yielded the same exact results.

Edit/ps - forgot the getvar all...

(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: 1.09.20.1112
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 6.20.651.3
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: **nah**
(bootloader) imei: **nah**
(bootloader) imei2: Not Support
(bootloader) meid: **nah**
(bootloader) product: m8_whl
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B70000
(bootloader) cidnum: SPCS_001
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 205bdca3
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
all: Done!

edit2/pps - I just followed the exact same steps on my s-off M8 and stock recovery is fine. (Had to flash twice because it included a boot image, but that's normal.)