I have a ton.
This one is more about my own foolhardiness, but I also learned a ton and came out stronger & wiser.

when I started out as a dev (my very first job!) and had not learned to say no.
I was a novice way out of my playground.

Like lifting a full booking platform from legacy php to Laravel & launching it like yesterday because it’s high season.
Didn’t know the full domain, so I just built something quite different in a week and shafted the existing db into it.

Obviously wasn’t feature complete or anything, so it resulted in maintaining legacy while building the new one and because it was already live and on different domains, we didn’t fully know which ppl went to, I had to every day painstakingly back port data from both platforms.
What I initially thought would take a few weeks that was launched in 3 days, spanned across 2 years plus one year refining and cleaning up my mess.