[AT&T HTC One (M7)] dborgo:
I just went through trying to get warranty service on a 1 month old HTC one, was refused, and am now frustrated with the whole process. This is a post both for me to vent, as well as a warning for all who are noobs, as well as seasoned veterans, thinking of unlocking, rooting, installing custom roms, and just in general messing with your phone.

I bought my HTC one online through AT&T about one month ago, beautiful silver 64 GB model. I had the customary 14 days to return it for any reason, except that date begins on the ship date from AT&T, not 2 full weeks in your hand. I heard a funny buzz from the top speaker within a couple days, but it was brief and I really did not pay much notice to it. As it was, it only happened at certain frequencies when playing music or videos using the otherwise awesome front stereo speakers. The source is definitely something vibrating under the speaker grill, but only when certain sound frequencies are made by the speaker. It is not the speaker itself, as I could cover that with a finger over the central speaker grill and it would then accentuate the vibrating noise by blocking the sound output, causing it to reverberate under the speaker grill even more.

As it would happen, I did not use the external speakers all that much during the 14 day evaluation period and did not really take note of it until about a week after, now past when AT&T would take it back. I waited another week in denial and then decided to contact HTC, since I was now keenly aware of it and could hear it almost every time I played music through the speaker. Now I don't post much in XDA, but I have been flashing ROMs for years and am well versed in the process. I lasted all of about 12 hours with my new HTC before unlocking the bootloader and flashing my first custom ROM. I had come from a long line of Samsung devices most recently, and although I never needed warranty service, I never worried about it because you could get those phones completely back to stock with no signs they had ever been tampered with. Not so with the HTC. One little line of text atop the bootloader screen would reveal my unlocking/re-locking.

The phone went back to HTC and immediately I had an email stating the phone was outside warranty, and asking for $200 to repair the device. Now I argued and escalated my case as far as I could, speaking to multiple reps, and eventually they offered to repair the phone for $100, only because there was some misunderstanding between myself and the very first rep I talked to when asking for warranty service (we had talked about whether HTC cared if the device was "unlocked", but the rep thought I was talking about sim unlocked and had told me HTC wouldn't care). I agreed to pay once they assured me my warranty would again be in force, as long as I did not unlock the bootloader. As far as I am aware, there is no way to unlock this phone and then return it to untouched factory status. It will always show that it has been tampered with. I was told by HTC that the main board would have to be replaced to remove this tampered status.

Learn and live, but a warning for all that go down this path. You play at your own risk. For me, I will run stock now for a good many months, until I feel sure the device has proved itself. Then my temptations to tamper will likely overwhelm me again....