I haven't found sizing all over the map, but I am peeved by vanity sizing. I used to be a waist size 29 in jeans, absolutely reliably. Suddenly I'm a 27 in some brands. Yeaaahhhh right. Nice try, joe's and seven for all mankind! I just ignore the number now and eyeball an item, try it, then go by what fits.
My saddest peeve is the brands I rely on suddenly rebranding and changing approach. For example, I used to love Rag and Bone jeans, but I think their new /jean line is almost insulting. The cuts and styles and fabrics are totally different from the older line. And not made in the States anymore, which I really appreciated. Why, R&B, why?