Seems sort of silly, but the one part of sewing that I just can't "get" no mater how hard I try is pinning. If I use them, everything just turns out worst. They get in the way, I don't know how to put them into the material, etc. I spend so much time fighting with them, 95% of the time I don't bother. I see them in photos all the time and it just makes me laugh (or maybe cry) because I honestly have no clue how to use those silly things....
I've made a down quilt, backpack and a huge 3.5 person 2# tarptent. And I have probably attempted to use pins maybe 4-5 times and in the end I always just give up and do without them.
It is one of things that keeps me from attempting to make clothing (that and the fact that the directions randomly leave out details). As it seems they always use a lot of pins in the process.