I purchased a dual-voltage step-down transformer. It had 3 input leads for either 115V or 230V and could output either 12V or 24V on the center-tapped secondary. I taped off one of the input hot wires and connected the other two to a wall plug. I then connected the three outputs to a screw terminal so I could safely test them with my meter, but they were otherwise unconnected.
I went to plug the transformer in (to 120V) and it buzzed loudly for a second and then violently exploded with wisps of some pleasant-smelling magic smoke. On the primary side I could see that it had spewed some copper from the primary winding onto the insulation of the wires (I could scrape it off to see insulation below, so the hot/neutral wires weren't shorted because of cracked insulation). The transformer was hot to the touch even after sitting unplugged for a few minutes. Testing the resistance of the primary now I get an open circuit between one hot and neutral and about 1 Ohm between the other hot and neutral.
Why did my transformer blow up? The first thing I thought of is that it was defective and the primary winding was shorted somewhere. Is there something else that I'm missing? Did I need to connect the primaries in parallel? The wiring diagram on the datasheet is very simplistic but doesn't seem to indicate such - it quite literally has 3 lines coming from the primary side that say 0V, 115V, and 230V.