I was looking at ATMEGA16U2-MU datasheet pages 186-188 very carefully and noticed that USB ground should be connected to UGND but also to overall ground of the board. But in Mega R3 schematic there seems to be jumper or 2 pads close to each other but no solid connection. There is a text "GROUND" close there too.
Am I interpreting this wrong and what is the purpose of the partial grounding? I have no board here so I'll have to ask.