Just got my car back from a mechanic who was putting in new wires and spark plugs... The car was backfiring as I was driving home. I pulled over several times, and couldn't figure out what was wrong. Finally, when I got home, I realize the 5 and 6 cylinder wires had been crisscrossed, so both cylinders were misfiring! How much damage could this do to my engine? Should I be worried?