A factory restore image is your way back to stock when you need it the most, so be sure to grab it and keep it handy
HTC has posted the 1.29.1540.3 factory restore images for the unlocked Developer Edition HTC One on the HTCdev site. Available in two flavors -- a zip file and a Windows RUU executable -- these files will allow anyone to restore their phone back to an out-of-the-box condition as long as there is access to the bootloader. That means no matter how bad you've screwed the system firmware up from monkeying around with it, you have an easy path back to stock settings. Then you can do it all over again.
Posting two different versions is pretty nice, too. The Windows only RUU is simple to use for folks running Windows (just plug in the phone and run the program), but not everyone uses Windows. For those folks who use a Mac or Linux computer, the zip file and fastboot makes it easy to go back without building a VM with Windows to do so.
Be aware that these images are only for the unlocked Developer Edition, so don't try to just flash them to your carrier model. Your favorite ROM developer will have something for you instead. It's recommended that everyone with one of the Developer Edition models have these files in a safe place "just in case", so grab the 980MB file at the link below.