I'm a full on impulse buyer. I once stuck a £100 deposit on a small fibreglass boat/barge thing when I was about 19, got home and asked my mum for the rest of the cash and she flipped. Had to go back and beg for the deposit back.
Bought umpteen bits off ebay as they appear to be cheap, only to find out that if I'd have shopped around a bit I could get them cheaper elsewhere.