Not taking my red devil's aggression seriously enough was probably one of my bigger mistakes. The less extreme example, he managed to bite me...which I've gotta be honest, was kind of awesome.
Unfortunately, his hate wasn't limited to me alone. He despised his heater. No big deal, he thwacks it until it knows who's boss every now and again, right? As it turns out, fighting back is an option for inanimate objects sometimes. He managed to bust the thermostat. I left for work with a healthy, if ill tempered, red devil. I came home to RD stew. Less than 12 hours between seeing him living and dead, but he smelled like he'd been gone for days.
Oh, and falling for the old "you can have my tank if you take the fish with it". It's a trap every time. Got a 55 with an adult red bellied pacu. Poor thing had to get really creative to turn around. Being a broke kid at the time, I couldn't afford to buy him a new tank. Best I could do was try to find someone that could house him while I did water changes so often that I felt like I was bailing water from a sinking ship.