Because my first car was a Supercoupe, most of my mistakes were the kind of stupid things that new drivers do. For example, the first time I "changed" its oil, I replaced the oil filter (but forgot to drain the engine oil). I added 5 quarts of oil (on top of the 5 that was already there). Thankfully, I checked my dipstick before starting the car and saw it was WAY too full.
I've snapped a lot of plastic trim pieces and mangled the crap out of my headliner (which needs to be replaced). I accidentally bent the cardboard backing as I removed it from my car (it's a moonroof, so it has narrow sides that bent easily).
Probably the BIGGEST mistake I made was buying my Supercoupe from a redneck that drove it like he stole it (and had crashed it into a pole while hydroplaning on bald tires). Then I spent thousands of dollars replacing the fenders, bumper cover, doors, trunklid, wheels & used tires, motor mount, transmission, rear differential, axle half-shafts, coil springs (the car was lowered too close to the ground), accumulator ball, and repainted the whole car except the hood. It still needs a new windshield (it's cracked), new interior (it's ripped up and smells like cigarettes), new headlights (the corner ones are busted), new power steering pump (LOUD NOISES), the radiator fan doesn't always come on (it's almost overheated 3 times), and the A/C doesn't work (not sure if I'll fix it), and I still need to paint the hood (It's a Cervini hood that needs some fiberglass patches). It hasn't driven since spring 2015. It's almost roadworthy, so I'm kinda excited.
It turns out for $3,000 I could have just bought a Supercoupe that looked ok and drove normally. Oh well
I definitely SAVED this car, because nobody else would have been stupid enough to save an MN12 that needed this much work, even if it was a 5-speed