I've run 2 overstocked tanks before. One because I didn't know any better at the time, and one out of temporary necessity.
Contestant number 1: 75 gal tank with wd built into back of tank (actual water volume around 63 gal). 2 puffers, each about 8" (one dogface, one stars&stripes), one big picasso trigger (about 6"), a 48" tessalatus eel, and a blueline trigger (about 5"). 8O What the hell was I thinking??? FWIW, I kept up on my water changes and kept the tank nice and clean, so the fish were healthy, if a little cramped. I still feel bad though.
Contestant number 2: 125 gallon tank w/ fluidized bed filtration. I had a golden guinea fowl puffer (10"), a common pacific stingray (about 10" disc), and a big mean tomato clown. Well an LFS near me was going out of business and was about to sell a 14" brown nurse shark hatchling to someone with a 40 gallon tank. 8O :? Seeing that my tank had a lot of open bottom for the ray, I took the shark temporarily until I could find it a better home. She was with me for about 5 months or so, and went from 14" to almost 20". Cutest, mellowest fish I've ever had. Man she was awesome. I eventually found some friends with an 800 gal tank to take care of her for another couple years until she was big enough to go to Shedd Aquarium in Chicago. Lost touch though and don't know how she end up faring in the new digs. I'll never get another shark and wouldn't have gotten that one except for the chance to save her. Glad I did, but cripes could that fish eat--a dozen 4" prawn a day when she was hungry. 8O I was doing water changes every week and still barely staying ahead of the nutrients.