I don’t think I have ever had a WORSE case scenario, but I have been in some pretty bad situations.
I think anyone who has been in the hobby more than 5 minutes has probably overflowed a tank before, including me, multiple times....
once a few years ago, when I was just starting to get larger and more complex fish, I put a bichir with some goldfish to grow out....(to my credit it went fine for several months, however now two of the goldfish have missing eyes...)
A few months ago my favorite flowerhorn bubbles passed away because of stupid flowerhorn genetic issues. That one wasn’t my fault but I really miss him. He had a good 2 years.
One time one of my oscars jumped out of the tank while I was feeding them and hurt his eye (I got him back in and he has fully recovered, no long term damage).
I had my 75 gallon tank give me a dripping leak. Luckily I caught it can got the fish out and a new tank in before the leak got really bad (although while carrying the tank out it still had a small amount of water and while carrying it the leak worsened and water went everywhere).
I once had a large tire track eel (10”) in a 20 gallon QT tank as it was the biggest QT I had at the time and he ate the other fish in the QT (10 or so mollies and platies) all in one night then proceeded to escape and pass away the following day.
Those are all the ones I can clearly remember, some of them my fault (like the bichir, and the eel eating the other fish. The lid was extremely tight though and even had 4 pieces of firewood on top so IDK how he got out), some of them not. But you learn from experience more than anything in this hobby.