Single pole switch has two hot wires leading to the outside lamp?

Today I was fixing up some of my old switches when I came across a single pole switch that had two hot wires connected to it. It was inside the house and it is for the light outside the house. There is no other switches around it. The only thing that is around it is a recepticle at the bottom of it. Now, when I... show more Today I was fixing up some of my old switches when I came across a single pole switch that had two hot wires connected to it. It was inside the house and it is for the light outside the house. There is no other switches around it. The only thing that is around it is a recepticle at the bottom of it. Now, when I went to test it to see if it was live wire. I took my tester (which is a cheap tester that only has a 220 V on it). I placed it like I always do on the screws of the switch. However, this time it caused a small arc flash and melted the tester pin on it. Obviously, I realized that wasn't supposed to happen. So I took a flashlight and checked the wiring. It showed 2 black wires on it. Now, from my understanding of electricity. It is supposed to have the hot wire and the neutral wire for it to work. I was wondering why did my tester cause it to make that small arc flash and if the two hot wires going to the single pole switch is a violation of the NEC. Also, my tester is not fancy at all. It doesn't have anything to adjust the Ohmes on it or the voltage. It just shows a 110V and a 220 V on it.
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