This is probably the most common sewing machine annoyance. It can happen for lots of reasons. Cheap nasty fluffy thread that has a tendency to shed fibres and have tiny knots in is often the main reason a lot of beginners unknowingly blame themselves for. Another rookie mistake is using a thick thread (this often includes some decorative or most heavy duty threads) with a needle that is size 70 or smaller. The eye of the needle is also smaller so the fatter thread hits more friction as it is being pulled through resulting in.... SNAP!