Well, when you hit the foot or plate you know something is not right. You've got the wrong part in, have it in wrong, something's not lined up properly. When you hit the pin you've deliberately left something in the way. I occassionally leave a pin in and usually don't have a problem. But, honestly, it's usually because I'm being lazy. If I really want to leave the pin in to make sure I get the stitch in the correct place I sew really slowly and take the pin out immediately before the needle goes in the fabric. Many years ago I sewed over a pin and it broke the needle and thread. I tiny part of the needle flew up and into my eye. It stayed down in the pink part of my eye-where you pull the skin down under the eye (if that make sense). I wore contact lenses and was used to putting my fingers in my eyes so I was fortunately able to get it out without too much trouble. I still prefer to wear my glasses when I sew, just in case!