When the timing is thrown off, the needle will no longer pick up the bobbin thread or the machine will skip stitches as you sew. Might do other things too, but those are the ones I know about. Timing is not something most people can do at home; requires an office visit to the sewing machine doctor!
I think the fine pins people are referring to may be silk pins. These are long, slim pins that do not have glass or plastic heads -- very plain. They are more expensive than other pins, but well worth the money IMO. They are made very fine so as not to leave holes in silk fabric. They are the only pins I will use for piecing. Even though they are slim and fine, I still remove them as I come to them and wouldn't dream of trying to sew over them. I have broken my share of needles and bent my share of pins trying to do that. Got scared when part of a broken needle hit my eyeglasses and vowed never to try to sew over pins again.