@OhEff think you win! Millions?! 
Mine are:
Marrying the ex, cost me in so many indirect and hard to quantify ways. Giving up my career to be a trailing spouse (he was army), the divorce, endless nonsense with contact, paying to facilitate his contact including hotel stays when dd didn't feel secure he would return her when he was supposed to and wanted me nearby, legal fees, unpaid maintenance - off the top of my head reckon the divorce and contact etc alone cost me around £100,000 add in the lost career and I dread to think.
In terms of things, mainly buying 2nd hand tech. It ALWAYS goes wrong and the seller ALWAYS pissed me about! Will never buy 2nd hand again I'd rather go without.
Op - update - yay! Slight win then
Train travel (this comes under the ex crap) learned that those advance tickets are NOT actually cheap unless you're only doing a short journey with one operator. I was travelling from one end of the country to the other diagonally! So at least 3 different operators sometimes 5. If one train (usually 1st or 2nd of the journey) was delayed and I missed the connection then I had to stump up to buy new tickets as they're specific to the train and time. The company with the delayed train isn't contractually obliged to ensure you make a connection so they won't accept responsibility, the company who's train you missed doesn't have to account for delays by other companies. This is why we need NATIONAL rail back! And even advance tickets are extortionate. It was most of the time much cheaper for me to - bus to airport this end, fly to airport nearest wherever ex was, ex more often than not wouldn't meet at airport if it was more than 30 mins away so then train/bus at other end and he'd meet us at bus/train station. Total Fucking palaver! I couldn't drive at the time as meds I was on wouldn't allow. Ex travelled to us ONCE!
and bitched about it for MONTHS! He got discounted travel and warrants etc but wouldn't use them to see his child!
Also I HATE flying Fucking terrifies me avoid it wherever possible snd ex knew this too. Bastard!
I didn't quite manage to put wrong fuel in car, but bought a car just after split from ex as he kept the family car. All pleased with myself I got a good deal. Got to the Asda petrol station, got out the car to fuel it and...realised I'd no idea what fuel it took! Completely spaced! Went up to the window to ask the 12 year old cashiers advice and she was just like
no idea! Car behind me bloke was getting impatient. Came up and asks what problem is thinking I've broke down or something. Explain and he smiles, "have you checked the book that came with it?" That prompts me to remember I even HAVE this! Go to look, book covers all models which means still no idea. End up phoning the dealership I've just bought it from!
impatient chap who I'm apologising profusely to is actually really lovely and says "don't worry about it. I'm still in the dog house with the wife cos I borrowed her car last week while mine was in for a service and..."
Yep
"...put the wrong fuel in AND tried to drive it. Cars a write off! I'm looking for a new one for her"
I end up recommend the place I just bought from 
I did once fall asleep on the train on the way home from work and missed not only my station but the guard had to physically shake me awake at the end of the line. Cost me £45 taxi when I could ill afford it. That literally cost me dinner for a week 