[T-Mobile HTC One (M9)] DunningKrugerEffect:
So yeah the people in the Int'l forums got that 1.40 OTA and the ROM devs have updated their ROMs to the 1.40 base.

Of course, this means we T-Mobile users are absolutely screwed unless we too update to the 1.40 base since trying to flash anything with the 1.40 base will make us bootloop and cry and swear. Mostly cry...and swear.

So here's a quick guide on getting your device to the 1.40 base so you can actually flash those ROMs again. Here we go. Note: I'm not responsible if anything bad happens. I sympathize, but it's not my bad.

Prerequistes:

1. You need S-Off. That means you'll need to fork out $25 to Sunshine and get it done.

2. You need Minimal ADB and Fastboot. The link for that is here.

3. An external SD card large enough to hold one nandroid backup and one ROM.

4. You'll need to make a Nandroid backup for your protection.

Guide:

1. First thing's first. You'll need to download ANY 1.40-based ROM. It doesn't matter which one, whatever one you want. Put that on your EXTERNAL SD CARD. You can find those in the Android Development section of the HTC One M9 international forums. The link for that is here. I highly recommend one that boasts that it's compatible with T-Mo devices. I flashed one of the Int'l-only ones and lost mobile data. Had to download and install another ROM. Learn from my mistakes.

2. Next you'll need to pick up the 1.40 firmware. The one I used is linked here. You'll want to download the latest one.

3. Now we get to have some fun. Open up minimal adb and fastboot and put your One M9 into DOWNLOAD MODE. Type "fastboot devices" without the quotes to make sure your device is recognized. If not, you need to go download the HTC One M9 drivers (just download and install HTC Sync). Once fastboot recognizes your device, type this command:

fastboot oem writecid 11111111

That is eight 1's, by the way. This will make your phone capable of flashing just about any firmware. Be careful and don't flash everything you find on the Internet. Once done, reboot back into download mode if it's not already there.

4. Rename the 1.40 firmware you downloaded to "update.zip" (no quotes) and put it in the minimal adb and fastboot folder (usually under C drive in the program files x86 folder).

5. Okay kids, this is the hard part so pay attention. You're going to flash the update TWICE. Why? I'll explain in the moment. Type these commands in order:

fastboot oem rebootRUU

followed by:

fastboot flash zip update.zip

Now at this point you'll probably get something like "90 Fail" or "Fail 90". What's just occurred is that part of the update was flashed, but not all of it. Sometimes it'll un-**** itself and continue on its own but usually not. To fix it, simply type the command a second time:

fastboot flash zip update.zip

This should complete the flash.

6. Reboot back into your recovery. Mount any partitions that may not be mounted.

7. Flash the ROM you downloaded back in step 1. Make sure you wipe everything (system, data, cache, dalvik) before doing so.

8. Reboot and enjoy.

Now that should be it. If you have problems, you'll want to visit this thread and the thread you downloaded the 1.40 firmware from (linked above) because those guys seem to know what they're doing and I really don't.

From this point forward, you SHOULD be able to flash any 1.40 based ROM without issues or cheap fixes (like in the ViperROM...by the way, it was really awesome of them to include that fix, even if it was a cheap fix. Thanks guys!).

Also, a big thanks to NO ONE for writing this guide anywhere else. Good job, jackass. As per the norm, you don't have to thank me and I don't care if you copy/paste the guide somewhere else without giving me credit. Information is free.