Which car purchase turned out to be a big mistake?

I once bought an '82 Olds Cutlass Ciara as a daily for my wife.  I knew they were junk but I somehow managed to convince myself that because this particular one was going to be an exception because it was low mileage and clean.  I worked on them when they were new and they were crap then so I don't why I thought this eight year old one would be different.

Sunbeam Alpine, lost my ass to make it go away.

My '13 135i. It was a clean, low mileage dealer maintained car. The Euro/Exotic Indy steered me toward on of those over an E92 M3.........told me the M3 would send him on a nice european vacation for what it would cost to maintain it. 

135i had 26k miles on it when I got, 32k when I traded it in. In that time it needed plugs, coils, VCG, OFHG, belt/tensioner, battery (which has to be programmed to the ECU because of how it regulator works), and the DCT reprogrammed 3 times. Then it developed a knocking sound at high rpm that we couldn't determine was coming from the engine or trans. 

I drove it down the street and traded it in on Mazda 3 that we still have. 

Outside of the maintenance, it put me about $6k upside down in the Mazda. 

I don't know I would consider it a BIG mistake, but in retrospect my MR2 was showing a series of red flags that I ended up paying for. I bought the car at night in an unlit parking lot from a guy who had only owned it for a few months, only took it for a short test drive, and gave the guy pretty close to what he was asking because the interior was so nice despite its high miles. Turns out it needed a lot of little things that I wasn't aware of, it was a little rusty, and the reported engine rebuild (with no paperwork) seemed less and less likely by the day. I had overpaid by at least 1k. I sold it a couple months later because I never fell in love with it enough to justify the time I would have to spend fixing it. The master cylinder failed on the way to meet the buyer, but miraculously he bought it anyway at a discount. Overall I lost a good bit of money on the MR2. I would say it's the only car I've had that I wouldn't consider "well bought," but it was still fun and I had a cool car to tool around in for a little while, so it wasn't all bad. I actually remember it pretty fondly, but objectively speaking the car was definitely a mistake to buy.

Oof. Probably my short-lived Fiat 124. 

How it started: https://grassrootsmotorsports.com/forum/grm/just-tell-me-it-will-all-be-okay-nd2-miata-as-a-da/180460/page5/

How it went:

Bought a one-owner low-mileage Chevy S10 Blazer.  Everything that could break did break. Biggest piece of junk.  

Boat tail Buick Rivera.    After washing Waxing and carefully detailing it.  I was sitting inside looking at how beautiful it was.  
    Suddenly the taillights came on. All by themselves ,  nobody near the car.   Yep the running lights too but not the headlights or brake lights.  
 After pulling the dash(4 times)  and going nuts trying to  figure  it out I found out the electric wind up clock would unwind and then trigger a solenoid to kick the winder around to wind it up.  
    The contact points ( made of lead) froze and melted a stream of lead across the printed circuit board.  Looked exactly like factory.  
 That was the easy problem.  
    It had a nylon timing gear to make sure people didn't hear it.   A corner of the nylon broke off and held the pressure relief open and zero oil pressure. 
    The condenser overheated and ignition went south.   
  Then the isolation plate underneath the Quadrajet melted and  the engine refused to idle with that air leak.   
   That's when I patched everything up. And traded it in on a new Chevy Vega GT that turned out to be one of the best Cars I'd owned. 'Til then.  

Jag XKR convertible. 

Same story as everyone else. Super low mileage, absolutely pristine, I thought "this Jag won't bite me in the ass." 

Wheel speed sensor goes out on the drive home. Thankfully it was easy to reach and just cleaning it fixed the issue. Various other issues left me stranded on the road twice. 

After about 3 months I got sick of it, several dealers were doing the "we'll match KBB" promotion. Miraculously KBB was a few thousand more than I paid for it, took it to 2 different places and neither of them would touch it with a 10ft pole.

Ended up selling it private party to a guy who flew out from CA. Nav stopped working for him somewhere in AZ, and I think something else broke during the road trip. Thankfully he insisted I take it to the local Jag dealership for a PPO, which I did. It passed their "inspection" with flying colors so when he complained I at least had a leg to stand it.

1989 Toyota truck with the 3.0. Figured the miss was just a spark problem. It had piston parts in the oil pan. Dropped in a reman engine and then it still didn't have enough power to pass slow cars at 6,000'. It was reliable other than the bad engine but it just wasn't up to the task even in full health.

Interestingly, it's probably the only car I've owned that I have few/no pictures of. That says a lot.

My A2 five-door Golf GL. I was just about to graduate from college and needed a new car as my Accord had just died. I was in a rush and paid too much. And sold it too quickly and didn't recoup enough. 

I didn't care for the Golf and sold it for a (new) Sentra SE-R.