New receptacle tripped/sparked
You have a metal junction box.  It is properly grounded.  

When you plugged in the tester into that receptacle, the receptacle moved a bit inside the box, and the screw came into contact with the side of the metal box.  Or possibly a ground wire was bent up somewhere it sbould not be.  

Solve it by wrapping the outlet in a couple turns of electrical tape, after screwing down any unused screws as Tester101 mentions.