I've messed around with various Arduino boards over the years but I never had an Uno R3 with a 16U2 chip. I decided to try some from Aliexpress (yes I know, not the most reliable but I'd had good experiences with other boards in the past).
A friend also got an Uno R3 in December as part of a starter kit. I got a similar kit in January same supplier. I got two additional R3 boards from a different supplier.
The boards in the starter kits were clones. The others had countefeit Arduino logos on them.
All of my boards didn't like some USB ports (previous boards had no problems with the same ports). Ports on a laptop and via a hub worked fine, others wouldn't connect and failed with a "device descriptor request failed" error. Uploading a few sample sketches worked fine though.
Next I ran few tests on the 16U2 chip. First I tried entering DFU mode: no joy, no reponse from "16U2" to reset. The friend who had the other R3 didn't have any problems.
I tried connecting a programmer to the 16U2 ICSP port: no response, can't even read chip ID.
Checking the 16U2 ICSP port showed several of the pins didn't connect to the 16U2 at all.
I strongly suspect the 16U2 chip is fake to reduce costs. Maybe a dedicated USB/serial chip or some other cheap processor. Most people using these boards wont know or care about DFU mode and will never notice any difference.
I got my friend to take images of the working board and compared them to mine (see below).
The area round the 16U2 is noticably different.
The most obvious difference is that the chip number is vertical on the good board and horizontal on the bad ones.

