Soldato
Porsche 911 - 15-20k, owner nearly 5 years but the mods were pretty much pointless now looking back.
I bought an Evo 8 new, back in the day. Spent a fortune on planned upgrades, tuning and a leather/alcantara interior and other bits all within the first few months of owning it. 10-15k perhaps. Did it make it a nicer car to drive? Nope. Did I waste all that money? Yup (accept the new interior). Was all a bit pointless but I couldn't see it at the time.
Now and then I get this tunnel vision and that's when I blow/waste good money and feel bad later. I did it again returning an E39 M5 to pristine condition. Funny thing was I enjoyed doing this more than I enjoyed just driving the car afterwards. Sold it for silly money in the end (too cheap). I think I'm cured of this now, until the next time.