Probably you would know why to flash the RUU. If not then know that from the following lines. Normally you would flash the RUU to bring the phone to stock settings.
When you flash the RUU on a unlocked bootloader you will give error and abort the process. In order to successfully flash you need to relock your bootloader first. Follow the given below steps for that.
DISCLAIMER : Best Android Blog would not be held responsible/liable for any damages that occur to your device by following the given below procedure. So, do everything at your own risk.
1. Turn off your device first, then press the power button and Volume button to get into the bootloader.
2. Use the Volume key to scroll down to Fastboot and press the power button to select it.
3. Open the command prompt and point to Fastboot folder.
4. Type in fastboot oem lock.
5. Finally reboot your HTC One X smartphone and flash the Rom Update Utility (ROM) it should work now.
For more information; here is the list of Rom Update Utility’s with different Mirrors