Constantly making small upgrades (then going through the trouble of reselling and losing money) rather than saving up and buying the best I could get at once.
Don't really have one for car audio, but my biggest audio regret was selling my Decware Zen amp. Don't know why I sold it...//content.invisioncic.com/y282845/emoticons/frown.gif.a3531fa0534503350665a1e957861287.gif
97 Hyundai Elantra hatch, gutted it, bought about 6 sheets of 1" MDF, built a floor, walled it, bought an 18" MT, 2 Colossus', 4 Kinetik 2400's, and about 100ft of 0 gauge wire.
Wish I didn't buy any of it, because all it did was drain my pockets and time.