My ECU registered a missfire the guy at the local shop reset the check engine and said that if it happens once the car will regester it and that it mostlikley only happened once should I be concerned by the missfire?? and what are some reasons for a missfire??

 

misfire can be caused by anything and everything check/replace:
sparkplugs
sparkplug wires
cap and rotor
injectors
fuel filter

etc...basically anything that makes spark/fuel

good luck