Great thread!!
I started updating my wardrobe about a year ago and went through ALOT of mixtakes.
1. Buying too much to soon - Now the reason why I did was to have a descent rotation early. At the I was going on vacation with my girlfriend and needed about 3 outfits. I almost fell for the Jos A Banks buy 1 sportscoat get 2 free scam 
2. Buying pants too tight to get the slim look - some of you guys seen my "How much should I taper" a while ago and seen that my pants fitted on the waist but were too damn tight on the thighs(http://www.styleforum.net/t/313607/how-much-should-i-taper-my-pants/0_30) . I bought about 5 pair of pants with the same fit the time(which goes back to mistake no. 1).
3. Not paying attention to details when dressing - learning various was to tie a knot,having a certain tie knot to a certain shirt collar type ,having a neat tie knot (tie dimples),matching lapels with tie width. There were times where I wore a skinny tie with a notch lapel sportscoat and vice vice versa. BIG MISTAKE!
4.Not tapering my pants if needed - I bought pants that would be covering about half of my shoe, which is a sloppy look in my opinion
5. Trying to put to much colors together - having a orange shirt, purple tie, and orange red pocket square lol. Clean and simple it good
6. Buying a sportscoat just for the color and not the quality and brand - I bought a Paul Fredrick sportscoat because it was forest green smh. I should have return it but never did. I spent a good chunk of tailoring just to get the fit right.
7.Buying Polyester pants - this is the latest mistake I realized I did.Luckily I only bought 2(about 30 bucks each) which can be replacable in the future. I prefer wool pants.More expensive but worth it in the long run.
8. Not paying attention to material of clothing - knowing when your buying Cotton,Linen, Polyester or Wool and knowing the difference.