I have an old HTC EVO 4G LTE Android phone. It is dubbed code name "jewel". I was flashing it with a CyanogenMod custom ROM following the instruction on CyanogenMod's website. However, once I finished the flashing process, and the phone stays in boot animation cycle, i.e., it would boot into the HTC logo, and the screen would turn dark, and the process repeats.
It turns out the boot image was not installed properly. To fix it, extract the boot.img from the custom ROM zip file, and upload the boot image to the phone, and boot the phone into fastboot mode, and run the following command,
fastboot flash boot boot.img
This procedure is discussed in an xda-developers.com thread.