This is a great thread. In the past year my mistakes were:
1. Purchasing too much of one color (I enjoy blue and grey, but really there are other colors out there!)
2. Duplicates of items (I've been guilty of duplicating not only colors but also items: 3 grey blazers, 4 black cardigans, etc)
3. Warm colors do not work on me, I have donated most warm items in my closet by now.
4. Seasonal prints, I shift seasons so fast during the year, I need items that are seasonless in print so they don't sit for 3/4 of the year.
5. Seasonless fabrics, similar to #4 above, I have to be careful of items that are truly for one season (heavy weight jumpers/sweaters, sweaterdresses, summer dresses), my items need more versatility. A couple of season specific items is ok, but not a whole seasons worth because then clothes just sit unworn for the bulk of the year.
6. Dry clean only clothes. I will dry clean blazers but tops and dresses that are dry clean only I rarely wear. I prefer the bulk of my clothes to be machine washable.