Today I had a problem while trying to return to stock my HTC One: I ended up having this HTC RUU Mode screen with warning logos on the edges and I found no way to get out of that; rebooting, even with VOL – pressed, brought me to that screen. I thought it was bricked, but don’t worry, there’s a solution!
My phone was S-ON and LOCKED, and it had a wrong CID: was set to HTC__405, which was not it’s original one, so I thought that there was a very little chance to recover it; however it can be done and it’s pretty simple (once you know what to do 😀 ).
I found out that, on that screen, fastboot OEM commands work. That doesn’t bring us directly to a solution, but it’s enough to start with!