The outlets power goes to ground when you plug something into it
First, those outlet testers are a mess in a dress.  Not the tester, that's simple enough, it's just 3 lights across the 3 pins.  But the legend, and all the silly things it says, is about as helpful as a magic 8-ball. 

The light labeled "open ground" measures across the power blades.  The other yellow light measures between hot and ground.  The red is between neutral and ground.  

You said you plugged in something, but didn't say what the load was. A little bit of voltage between neutral and ground is actually OK.  Neutral is not ground.  If such a low voltage sets off your tester, it's too sensitive.  Otherwise I'd look for a wiring problem in the neutral.

Are you using screw terminals, backstabs, or poke-and-tighten-screw?  Are you doing it monkey tight, not gorilla tight?  

Just so you know, people, especially inexperienced people, find backstabs are not reliable.  In many circuits they are not even legal.  I avoid them entirely. 

Sometimes, the screw terminals on the side of a receptacle will strike against the side of the outlet box, or hit the bare ground wire.  Often this will only happen when the weight or strain of a cord is bending the outlet. A cure for that is to wrap several loops of electrical tape around the perimeter of the outlet, i.e. Over the screws.