The most expensive mistake you’ve made on a project car?

Ever time a Ford V8 off of #5 cylinder because you had just worked on a bunch of Chevys?

The supercharged engine did not like that very much.

^ It almost always has to do with firing order and things like "does it sound like it's misfiring?" 

Still not sure if it was not enough fuel, too much ignition advance, or an assembly error, but I had a freshly rebuilt 4AGE lunch itself one time, and not back when they were common, but into the era of 4AGE parts being hoarded collectibles.

Another time I accidentally put brake rotors onto hubs with a center bore flange diameter that I didn't realize was 1mm larger than it should've been. Felt a little snug going on but nothing major. It cracked the rotor hats wide open. They weren't super fancy racing rotors, but they were ordinary rotors sent to the Caribbean with hugely expensive shipping and massive import duties, which was almost as bad.

Ripping a CV boot because I'd forgotten one step during a LCA bushing job on a friend's car - popping the lower ball joint.  It's one reason why I prefer the owner of the car not "help".  They are often a distraction. 

Having to have my ex's Volvo wagon towed to a shop when I simply could not figure out how to get the timing belt off the lower pulley of a Volvo T5.

Does "buying a bunch of parts that you never actually use because you change your mind by the time you get there," count? Man I have a lot of those...

Biggest mistake was buying it... Sigh...

God God...still waiting for it to end!

Bad AOD led to two cam shaft changes due to low vacuum that led to three gearbox changes and three fuel mixer-system changes  and three manifolds before I figured out that it was the distributor and not the fuel mixers that were the issue.

Other than the fact that I am now past all that, I think Ford V8 swaps are the most horrible things in the world.That said, I would not have learned a lot had I not gone through the maze.

I was having problems with the car cutting out at a rallycross one day so I whipped out the laptop and got Tunerstudio fired up.  I decided that the problem was that the overboost protection was kicking in for some reason.  So I went ahead and turned that off.  Problem solved!  It didn't cut out anymore.

Except, of course, that the overboost protection was kicking in because the car was, you know...  Overboosting.  The boost control had failed in some way that was keeping the wastegate closed.  The car ran absolutely amazingly until it cut out one last time due to an extra hole being placed in the block during an escape attempt by one of the rods.

I now have a strict "no laptop at the race" policy because I can't be trusted.

Destroyed the pump on a DSM 1g AWD auto trans trying to get the converter in place. The trans was specific to 91 only. I bought a 92-93 trans and the electronics were different. I wasn't smart enough and the info wasn't online at the time for me to fix it. Begged for help on DSMTuners with zero responses. Sold the whole mess for a massive waste of time and money. 

In reply to wae :

I found out that Evan had the overboost protection somewhere around 20-24psi when I went to borrow it one time, stopped for fuel on the way home, and on the onramp in 2nd gear, maybe half throttle, I saw the boost gauge bend its peg as the engine cut out.  Not a good thing to feel in not-your-car, at night, 20 miles from anywhere.

the fishtank boost controller had rusted shut.

Engine was fine, but yeesh, dat boost cut was the scariest thing.