For me it would be the 200 euros I spent to re-buy a plane ticket after narrowly missing my flight by about 5 minutes (which I still maintain wasn’t my fault as the passport control line was like two hours long)
I’m sure other people have wasted more money on tough life lessons so let’s hear it 
My divorce. When we split up, my wife was concerned with how the kids would handle it. So, we had a verbal agreement that I would basically give her the $$ I had been making from my job and I got a second job for my expenses. Then, when both kids were >18, we would divorce amicably.
The verbal agreement was worth the paper it wasn’t written on. When the kids were both >18 she got an attorney and gutted me.
Worth every penny, but OMG the pennies.
Lent $1,100 to a friend, was never repaid.
Put $1,400 into a healthcare reimbursement account, thought it was something I could simply put back into my normal money if I didn’t use it - nope, it’s nonrefundable.
Car accident, turns out I had failed to buy collision coverage on my insurance - cost $2,500.
Bought some property (I was 22-ish) then decided I really didn’t want it, so I contacted a realtor to offload it for me. I was impatient, and stupid. I quit making the payments and finally gave it back to the developer in lieu of foreclosure. I figure I’d made at least $5K in payments, all gone. Had I held on a little longer, I know now I could have sold it for a profit. But I was young and dumb. Expensive lesson.
A couple I know have had a tumultuous marriage. At one point they moved from Pittsburgh to somewhere in Florida. Their Pittsburgh home was worth ~$180,000 and was owned free and clear. They left the home sitting empty. They took out a second (first?) mortgage/home equity loan to help with their move to Florida, but once they got to Florida they split up for a few years.
Neither of them felt they were obligated to deal with the home in Pittsburgh, or the payments on the loan. They eventually were foreclosed on, never dealt with that either. It was a really cool house.
We had a house foreclosed. Long, pitiful story, I won’t put everyone to sleep.
But that humongous real estate boom about 15 years ago? As soon as we signed the listing agreement with the Realtor–the ink hadn’t even dried on our signatures–the boom went bust.
So we lost the house, our credit was dirt for seven years, and the saddest part is that it was the home where I grew up, and then later raised my own kids. I felt like our losing the house was a slap in the face to my mother’s memory. That part still hurts.
~VOW
Bought a timeshare for about five grand. Never used it and the exchange company never dis come with one we wanted,
I once paid about $40 in Portuguese escudos in Lisbon for two glasses of lemonade. Didn’t check the denomination closely, even though I was living there at the time, and the waiter didn’t bother to tell me I’d made a mistake. Been lucky with money overall, I’d say.
Every bet I ever placed (with a bookmaker, I mean) lost.
I’m really not sure that this counts, though. There weren’t many (though it obviously was dumb); and it certainly didn’t attract me to gambling, so worse outcomes were possible.
j
I had a problem with my front door, so I thought I could just make a small adjusment myself, (I have no handyman skills but it didn’t seem like a complicated job) My adjustment didn’t fix the problem but it did create a hole. Instead of getting a new door I just got a guy to fix the original issue, (costing £100) Months later after mice were getting in I had to fix the whole door for £1000, (that wouldn’t be an issue if I’d just ponied up the £100 in the first place.)
So £1000 worse off than if I’d just got a guy to fix issue 1 straight away and £100 worse off than if I’d accepted the inevitability of a new door after my bodge work.
$600 which was our rent. I think it was stolen but who knows. Back then it was a huge amount of money.
Every buck I’ve gambled has been lost. I luckily learned early and didn’t lose too much. (A sign: handheld poker game balance always goes to zero.) Most I ever lost was loaning a couple hundred bucks to my platoon sergeant who refused to repay before my discharge. I yelled at the other troops to never trust this thieving guy. Otherwise I’ve mostly avoided losing money. Wouldn’t be prudent.
My SIL made a bad move, skipping on house payments because a “mortgage consultant” said it was a good idea. It wasn’t. Duh. He kept the house but it COST. Moral of the story: financial tricks that work are probably illegal; it legal, they don’t work.
The worst thing that ever happened to me was being a winner at casino gambling for about 10 years. Hit Vegas or AC once or twice a year type of thing. Nothing huge. Win 1-2K. Not a whale. Not living at the casino. Just a guy making some extra coin at poker/blackjack/craps a couple of times a year.
Then, winning $22,000 in about 5 minutes.
You then get on the “preferred” lists. “Free” rooms. “Free” meals. Matchplay. Etc.
Believe me, the amount I lost over the subsequent 10 years more than offsets the winnings. Way more. You convince yourself that you can beat it (hell, I did for 10 years). Lose at the casino? I can certainly win it back sports betting. I watch a lot of sports! How hard could it be? You start chasing. And chasing. And chasing.
It ends up bad.
Put my card in the cash machine. Cash machine spits out $300. Take card. Walk away.
That may not be the largest amount mentioned in the thread, but for “dumb” it’s pretty hard to beat.
The best I can reconstruct what happened, I had about $1,800 in blackjack winnings on the table. Was quite drunk. Went to restroom and instead of going back to table, decided I should just head home. Driving drunk was the dumbest thing I did that night. Leaving $1,800 on the table comes in second.
I’ve lost more money at times, but never doing anything* that* dumb.
About $4,000, due to a moment of inattention that resulted in my getting in an accident in a rental car. I had declined the supplemental coverage. Ouch.
Also, back in college I spent $250 or so on a dress to impress my then-boyfriend, who said only, “Don’t you think that was designed for someone bosomier than you?” I guess I can’t call it a loss since I still have the dress, but I deeply regret both making the purchase and not dumping his ass immediately.
Oh! And probably another $250 or so, in three different currencies, when my purse was snatched in a church in Galway. It also had a flash drive in it, which apparently got dumped by the river, because a nice fisherman found it a year or so later, found my CV with my address on it, and sent it to me by international mail. I feel like it was almost worth the $250 just to have that happen!
I agreed to rather generous monthly support payments to my ex-wife in part because I knew you could write off alimony if you itemize your taxes. I learned too late that if it’s not written into your divorce agreement, it’s not alimony and therefore not deductible.
At least it was only 5 years.
$600 on a fraudulent eBay purchase and never received items. It was the only open and blatant theft I’ve ever had with eBay and was an expensive lesson. Looking back it was obvious but it was a good price and I was a newbie schmuck.
$28k Capital Gains from a house bought and sold in 18 months time span. Could not buy a new (tax) qualifying property, and so was taxed on the gain. Ouch.
Oh it’s still going on. My mother died and left a bunch of stuff at my aunt’s house. I told my aunt to put it in storage and I would come get it…eventually.
Unfortunately this would entail going back to Oklahoma, so I never did. So I am paying for this rental unit, for decades. It’s not that expensive, but it just goes on and on.
I know I should just say, the hell with it, and somebody gets a bunch of junk and some Desert Rose china when they auction off that storage unit. I know most of it is stuff I don’t want, all of it is stuff I don’t need. A hope chest that my father made for my mother, the flag that was on my father’s coffin, and that china, that’s what I want. Obviously I don’t need any of it as I’ve gotten along without it all these years.
But I keep paying.