Yes I have.  When we first started trying to work online my husband purchased a website hoping to sell electronic products.  There is a lot involved with such a project.  After a certain period of time I convinced him it wasn't working for us.  By that time the money had been spent and there was refund.  Lesson learned.  Not just wasted money but time wasted too.

I have lost count on them but the latest one is. I booked two tickets from Canberra to New Delhi via a reputed booking agency in the month of November 2018. To our surprise we were informed by the airlines in the 3rd week of January 2019 that  we should pay the airfare immediately to keep our booking intact. We tried to contact the agent by mail but it was bounced back as they had closed their office. Later we found out that they had taken refund of thousands of bookings and declared themselves as bankrupt. There is almost no chance we ever get that $1300.

I have without a doubt wasted a lot of money in my forum's management. But you can't always make great investments.

I've wasted money on a few forums in the past that never quite made it. I paid for hosting and a domain name, but the projects ultimately failed.

I've bought a multitude of "premium" add-ons, only to then not use them for one reason or another..

I probably still have various vBulletin related licenses that I haven't used in years and never bothered looking into selling them (vBSEO, vBadvanced), as well as some XenForo add-ons that I've opted not to use since purchase.

Yes i have, i bought two second hand license of Xenforo (one after the other) then closed the license without giving a second thought. 100$ went just like that due to my stupidity and no second thought.

Over the time I've spent messing around on website projects I cannot count the amount of money I've simply just thrown down the drain trying to get things done. It's ranged from trying to game my sites SEO by buying copious amounts of "1000 BACKLINKS 100%" gigs on Fiver only for Google to strike me down to getting a custom theme made for a site which I had to close down not so long after. It's part and parcel I think haha, as long as you learn from it for the future!