It looks like the Use Multiple Serial Ports on the Arduino Mega page has a small error in the schematic example.

The TX pin of the mega is going to the TX pin of the Serial-enabled device, as opposed to the RX pin of the Serial device.
The RX pin of the mega is also going to the RX pin of the Serial device.

The deleted asset in commit 4aed0c1 appears to be the right wiring.
See #2275