I think my worst mods were just poor planning... I spent a lot of money on stuff I just threw away once I got a little more serious into the build. Having a solid build plan is the best mod no question!
I bought my Stang 5-1/2 years ago bone stock. My plan was simply to make upgrades to any areas where there was room for improvement so that the car would be more fun and reliable to drive every day.
It was never intended to become a dragster, track car, show car, or garage queen.
Any mod I have bought and then sold the car before installing.
This happens way too much.
I had to give my CC plates to the guy who bought my GT350. I still have a set of GT500 mufflers that I'm about to throw out. I have a set of new rear rotors for a 2017 GT taking up space. A set of shift bushings I bought for my RS, but my wife will get those to install on her Fiesta ST. A lower temp thermostat that I never used. I still have some track pads for a Focus ST...
I honestly have no idea how much **** I still have for cars I don't own.
BBK longtubes on my '97 cobra. don't get me wrong, they were cheap and sound awesome, but had i done my research, i would have bought a brand that doesn't require you to remove them to replace the clutch
I remembered another mod I removed. I originally installed UPR ProSeries Adjustable Control Arms with spherical bushings. The did take a lot of flex out of the rear end but... I immediately started hearing gear noise from the rear end that I had never heard before. That is probably OK for a track car but very annoying in a daily driver. It was the spherical bushings in the upper control arm. Metal on metal they didn't buffer out a single sound. I replaced the UCAs with Maximum Motorsports FRPP (Ford Racing Performance Parts). The gear whine immediately went away.