I recently tried on two tops at Old Navy, both in my favorite color amongst their selection:
The top on the right was HUGE. I don't mean one size too big for me. I mean I tried on their smallest size - XS - and it was still huge. Everywhere. Believe me, I am NOT an XS. The top on the left fit at my usual size, M. It also fit well at the shoulders, sleeves, boobs, everywhere.
I understand they are different shapes, but still, the same size should have fit me in both shirts. The top on the right should have fit a little looser throughout the body but the shoulder/upper sleeve area should have still fit me well. It didn't.
I ended up getting the top on the left, in three colors. Plum, taupe, and black. Like I've said before, when I find something I like, I get it in a few colors.