Should never have donated my 66 Chevelle for use as a stock car.
Should have chased down the $3000 Lotus 7 basketcase I heard about in college.
Should have bought the I4 instead of V6 in my '03 Tacoma (Iraq mess was already on the horizon)
Most egregiously, should never have bought the '97 Chevy Silverado (very reliable truck, has had major problem every 5K miles since almost new).