Ground, common, and Earth ground confusion
It's the most common mistake, almost an universal one in almost every schematic, to draw the reversed pine tree for GND or COM while it should be reserved for EARTH.
Normaly for GND/COM they should draw a solid triangle, preferably with the mention "0V".
In this context of electronic communication this symbol means GND.

Things get funny when you add TVS diodes or GDT's which do have to connect to the real earth. 
Then people should somehow think that something was wrong with their symbol...
You will also see some fork shaped ground symbol, which means chassi ground. Neither earth nor GND technicaly but often refering to GND too.


More about it here