I tried a loopback test yesterday on my (genuine) UNO, and despite having everything setup as it should be (RX connected to TX and GND to RESET) I don't get an 'echo' on my computer. I have tried two different laptops, three operating systems (Ubuntu, Windows and OSX), two USB cables and even tried replacing my UNO with a Nano!
In each of these cases, I am able to receive data over serial from the Arduino if a sketch is running, and can setup a sketch that echos all received data back as well, so there's clearly nothing wrong with the hardware. When I send data to my Arduino (in loopback mode) both TX and RX flash, so the connection between the two is fine too. If I connect my Nano's TX to my UNO's RX (or vice versa) when my UNO has its RESET grounded, any data sent by my Nano reaches my computer perfectly.
I have tried every test I can think of and, so far, every one of them has indicated that my Arduino(s) are perfectly fine. So why is my loopback test failing, when the Arduinos both work as expected when doing anything else?