[T-Mobile, Samsung Galaxy SIII] Gibson99:
ok so i took my t999 (plain old d2tmo), which had never been rooted or anything. did a factory reset on it to wipe everything, so i'd be starting fresh with stock 4.3. the skipsoft toolkit on my win10 preview machine and installed twrp 2.7.0.1 (i think) using that, and then did the trick to make recovery stick after reboot, installed supersu, and all seemed well. so then i copied a couple of roms to an SD card as well as v2.2 of this multiboot recovery. put the sd card into the phone, rebooted into recovery (standard twrp), flashed the multiboot, rebooted back into recovery, and it looked good.

i don't think it should matter, but i'm doing all this with NO sim card in the phone. i've done that before with others so figured it'd be safe here too. that way i can keep using one phone for work and get this one all set up and configured so that i can simply pop in my sim card and immediately start using it, with almost no lapse in coverage for work.

created a new rom on internal storage (~11gb free), named it "cm12", gave it the default sizes, "set" the rom so it was listed in red at the top. everything looks good so far. went back to twrp home, tap install, chose the cm-12-20150409-unofficial-d2tmo.zip from /external_sd/cm12/, and after about 20 seconds or so, it failed - error executing updater binery in zip '(path and filename of the cm zip)'. error flashing zip (path and filename again). updating partition details... ...done. then it shows the home, wipe cache/dalvik and reboot buttons. probably the weirdest part is that i can click back, and it should show a file listing of the cm12/ dir on the sd card, but it's just blank - not even a button to go up one level. have to reboot to recovery again to be able to browse again.

so i said, ok, maybe the download is bad or there's a major bug in that nightly download. i deleted the cm12 rom, made one for cm11 (again, default sizes) , flashed the cm11 M12 snapshot (just cm, no gapps yet) and it says success! so following the video tutorial, i went back into the multiboot menu, set rom, reset to internal, boot rom, cm11, swipe to confirm, then reboot new rom. again - big problem: when it reboots, it buzzes briefly and in the upper left i see blue text: RECOVERY BOOTING for a split second, then black screen. nothing at all after that. left it a couple minutes and still nothing - had to do a battery pull. after that powering up normally (just power button) automatically boots into multiboot twrp.

so i deleted the cm11 rom i'd created, then rebooted again just to make sure i could still boot to the stock internal rom. this seems to work fine - i got back into stock rom without an issue.

so, what am i doing wrong? i read this entire thread and watched the videos in the first post, but maybe i missed something important. this isn't my first samsung phone to flash - i also have a relay (apexqtmo) which a lot of its roms borrow pieces from S3 roms. it doesn't have multiboot, but cm11 m7 (one of the last stable releases) is pretty rock solid, so i haven't needed to switch roms.

i haven't tried using a standard recovery like twrp (without multiboot) to flash something over the stock rom yet. i liked having dual boot on my old droid4, and i figured since i had some extra room on this s3 (i have a 64gb sd card so i don't keep anything on internal), i could dual boot again to be able to play with roms and still have stock to fall back on if needed. so far i'm not having any luck. :(