You don't have to unhook the batteries to replace the injector O-rings. I have no idea where a spark could have come from. Even if using a metal "prong", you would have to touch something with power going to it with the key in the off position in order to ground out on your injector. I use a metal pick to remove the injector o-rings and never had any sparks happen. Depending on where, exactly, the spark happened on the injector, there's an extremely small chance that the spark could have damaged the injector. Maybe, possibly, if the spark happened inside the hole in the very top of the injector (where the fuel goes into the injector) then there could be a small bit of metal that is either partially blocking the hole, or it could have come loose and gone down inside the injector. Either way, it could be blocking off part of the fuel resulting in a miss. I will repeat that this is an EXTREMELY unlikely thing to have happen. If it is a bad injector (for whatever reason) then your cylinder is probably fine, but damage may happen if I'm wrong and the injector is actually hanging wide open. That would eventually wash all of the oil off of your cylinder wall, which would cause scoring and you'd have to have machine work done. That is if you keep driving it with the injector hanging wide open.
