Bought a punching bag and one of those dangling fast-moving punching bag thingies. Used it seriously for about one week.
Bought a Compaq computer instead of building my own: had integrated graphics + sound. Then tried to use that motherboard and case as a basis for building my own machine, integrated graphics/sound kept fighting my installed PCI card versions of the same.
Bought a JVC Everio MG-555 video camera: horrible focus, artifacts galore, screwy color. Sold it for half price and then bought slimline PS2 with 10+ games (that wasn't a mistake

)
Bought one of the world's only bluetooth trackball mice, "The Ball." That was dumb, the thing is completely worthless; not good as a trackball at all, very awkward handling, limited buttons, too small for my small hands, incredibly stupid and "slippery" charger dock (try to put it on the thing, you have to get it perfect; if you bump your desk slightly, it falls off the charger). It feels like you're trying to line up the air craft carrier landing on Top Gun for the NES.
Bought a 500GB USB external HDD instead of a FireWire one.
Bought a USB TV signal receiver dongle. Very rarely managed to catch an incredibly stuttery two channels. Sold it for half price (yeah!)
Bought a cheapie wooden desk with one shallow middle draw for ¥10500. Next week saw a much larger, nicer desk with more space for ¥7500.
And, more recently, since I'm still a "young adult," I bought a MacBook. No, don't get me wrong, I love it, but within a couple months I started seriously getting into video editing, and then HD video editing. MacBook just can't handle it well, and the screen is too small. My color correction/grading program refuses to even run. I should have purchased an iMac!

But, now I have! I'm waiting for it to arrive, and it's one purchase I won't regret. I'm selling my old iBook to help defray the costs, and giving my MacBook to my wife, and then taking her iBook (yeah, it's an interesting trade/sale process). Figure the wife won't get as angry about my purchase if she gets a nicer laptop out of the deal! Plus, she can finally play Halo on Windows!