In addition to what the other person said, (in the US) if the connection was made between sockets on different circuits, the voltage potential between the two hots could be 240v, making the hot wire a huge heating element, thus the fires.

As I understand it, one typical use case for these things is Christmas lights, and getting confused by a big tangle of light strings makes plugging in both ends in very different places an easy mistake to make.

Also I wouldn't take for granted that your outlets are wired correctly.