testing my 2240

[nl12]

I was testing my 2240 on the bench this weekend, I was testing with 8s lipo and MGM 16024, all was going fine I programed the controler, tested forward, reverse, started rolling on the gas a few times, everything fine, until I gave it a sharp jab of the gas and a spark came off of the controler where one of the motor wires is plugged in. After that the 2240 was in error mode, so I unplugged everything turned it off cleaned the connectors that go into the controler, the one that sparked looked a little black in some areas, plugged it all back in turned it on and it worked perfectly. Why would it have sparked like that? Would a little flux left on the connector cause it? I just want to figure out what happened so I can correct the problem so it dont happen again.
Thanks in advance.
Nick