About 10 years ago, I used to have a 75g aquarium with an oscar, three piranha, and an aligator gar. my initial mistake was misinterpreting the 1" per gallon rule, my second mistake was trying (unsuccessfully) through denial and error, to find another tank mate for that combo (could've sworn the bullhead cat I caught in the river would make it). and thinking i could feed a pinky mouse to the piranhas (they didn't really go for it, except for like one initial bite), that was horrible. poor baby mouse.
I made quite a few big mistakes with my first aquarium.