I always start out crossing a few lines ON PURPOSE, that way I can just say they were all intentional. I think the idea with stippling is for all the lines, loops, humps and bumps to have about the same distance between them. If your lines cross, then obviously they are "too close" together. As you practice and improve with stippling you may get bored like I did of just doing wandering lines and loops. That's when you start throwing in leaves and stars and flowers, butterflies, hearts... your imagination is the limit.