An Xbox 360 can show an E09 error on screen (at boot) which indicates a Hardware Failure of a kind. The E09 error will instruct the user to contact Microsoft Customer Support. It will be accompanied by a single blinking red light in section 4 of the ring of light on the console (as shown). There is some speculation as to what actually causes an E09 error, but there have been hints dug up from user reports and other sources.
One source of an E09 error is believed to be the GPU. I mention this first because I received an E09 error fixing an Xbox 360 console before when I put too much pressure on the GPU heatsink (I didn't have it bolted in yet, I was testing a boot) and thus putting too much pressure on the chip itself. While the E09 is associated with a GPU problem, it also has been associated with the DVD drive (which could indicate the error is actually attributed to a different piece of hardware on the board that is affected by either).
It is reported to be shown sometimes in cases such as a bad flash of a DVD drive, and has been attributed to the Southbridge on the motherboard. Either way it is not fully known by enthusiasts how a user could go about fixing this problem.
The error code associated with E09 is 0021. To get an error code from an Xbox 360 console that is displaying an E09 error (or 3ROL) you simply press and hold the "sync up" button (used to connect wireless controllers) and then press the Eject button for the DVD drive. This will bring up the first digit of the error code (usually it is all four lights blinking red, which represents 0, not 4). With the sync up button still held, press the Eject button again and you get the second digit. Do this twice more to get the full 4 digits of the error code.
For more information on the error code you receive, or on Xbox 360 hardware problems in general, read this Xbox-Scene forum thread.