It depends on how you connect the test leads to the RX. If you are manually holding/touching the test leads to the exposed header pins, and most header pins for JR connectors are about 2.5mm apart, it's easy (at least for my unsteady hands) to slip and have either the probes touch each other or one probe touch both pins, creating a short between the 5V and signal pins.
Something along the lines of this:
https://www.ibiblio.org/kuphaldt/electricCircuits/DC/00344.png
Ohh I got what you meant. That makes sense, yeah! At first I thought you meant, that If I measure lets say 5V pin and touch both of the testers together.
Either way now I got what you meant!