The way most people mess it up is because they have the presser foot down which tightens the tension discs. On newer machines with the tension discs housed inside the body of the machine it is difficult to see this. On an older machine you can actually see the disc come together and open when the presser foot is lowered and raised. It actually is a good thing to take a thread put a few knot along it and slide it back and forth through the tension discs (presser foot up) to clean out lint build up and keep the tension cleaner.