I suppose it could be difficult to call it a mistake, seeing I really don't regret it due to what I learned, but it's possibly the move that managed to change my life in a manner that cost me the most money.
I got a credit card with my first job. The credit card had a 0% interest on balances below $250. My maximum balance was $120. This in my eyes was a bit of free money. I paid $40 per month because I was trying to learn how to be financially responsible and didn't carry the maximum balance.
They raised how much I could put on that card from $150 to $950. Great! I upped a little bit of what I was borrowing, and upped the amount I was paying (still roughly 40% of the balance I carried). As time went on I started being a little bit more reckless. I knew I paid about 12% interest on my balance, but I was paying above the minimum and wasn't too worried about it.
I lost my job. I paid the bill no problem though seeing unemployment was more than enough to cover it. They upped the amount again, this time to $2500. I got a new job and made less than I got on unemployment. Not good, but at least I was having no problem paying the bill, still above the minimum.
They upped it again, just in time for my laptop to die. I purchased a new laptop on my credit card because I needed it for college. I had lost the previous two jobs due to severe knee pain preventing me from standing for as long as they required and began working freelance and working towards my degree. They upped my spending limit again. I now had access to over $5000 worth of credit, but essentially had no income. My interest was raised to 29.9%, even though I never missed a payment.
Time went forward like this, I was scraping by paying the minimum payments. They kept upping my maximum even still. As I got tighter with money, it became more tempting for me to use this line of credit to be able to eat and pay bills. Of course, this created a cycle where I'd use it to pay bills, and struggle paying bills because I used my card to pay the bills I was struggling with.
Come December 2014, I'm still unable to find work despite now having a degree. I ran out of options to pay my bill, I had never missed a payment despite all my hardship, and I just was no longer able to do it. I had no option but to admit defeat, and talk to them telling them I honestly couldn't afford to pay anything. The creditors started lecturing me and asking questions over the time I couldn't pay: "Can you borrow money to pay it?" (No, I have to borrow money to keep the utilities and rent paid, food is a greater concern than paying this bill, and I can't afford that) "Do you have an expensive phone with a data plan you can cancel?" (No, I'm on a $25 prepaid plan with a phone that I bought two years ago from a friend). What I owed kept going up, and it pained me to not be able to pay it, but I did what I had to do.
I finally obtained work in May 2015. I wasn't making a whole lot of money, but I started being able to pay this credit card again just before it went into collections. But, of course, I racked up a good few hundred, if not a thousand extra dollars in extra charges.
I do not regret this credit being available, it saved me from homelessness. But, by far it's the most expensive mistake I've made. I don't even wish to calculate how much money I'm paying out to get out of this hole I ended up getting myself into. But, there's a light at the end of the tunnel, and there's only 43 more months until I'm done with it.