I added 2 new wall sockets starting from an existing wall socket. After wiring them up and connecting the power I plugged a GRT-500 into the two sockets - I happened to have two so I plugged one into the socket closest to the starter and the other at the end of the line.
The socket closest to the starter socket read "correct" but the last one read "open ground". I shut off the power, inspected the box and receptacle but found no problem. I checked all the boxes I had opened or worked with, again finding no problem.
After failing my last attempt, I removed the socket testers, and just happened to remove the socket tester closest to the starter socket first -- when I did this, the socket tester in the other wall socket switched to "correct"!
I then verified that each socket "individually" reads correct, but only when there is no other tester ahead of it... Is this normal?