I've had a few regrettable purchases. By far the most regrettable were the V1 Tannus inserts. Absolute trash. I experienced 3 flats after riding probably a combined mile of trail running normal pressures and 1150 gram rubbers. Inserts removed immediately afterwards.
Kenda DH Nevegals and IRC Kujo DH tires were frightening, but then again anything without Maxxis or Michelin on the sides was back then.
The Thomson 1.5 X4 stem was a splurge purchase, but any forged stem would have been better. The stem required super high torque to hold it in place. The screw heads were tiny, and underneath that beautiful anodize was some pretty poor machining of the steerer bore which compounded the challenges with keeping the handlebars straight. That was a heck of a dud product as well.