There was one time I bought a peacock eel and after acclimating it to the water I took it into the kitchen to dump the water and fish into a net. Well he jumped out and went down the drain and into the garbage disposal. I spent the next 15 minutes trying to reach my hand down there and fish him out. After pulling him out I was sure he was dead but I stuck him in the tank anyway, he just came to life and swam under a rock.
My kuhli loaches were a pita when I had my densely planted 125 gallon. I had one side of the tank completely covered in blyxa japonica and they stayed in there and wouldn't come out.
One of the hardest non bottom dwelling fish is when I had some leporinus fasciatus. They are so damn fast and jump like theres no tomorrow. Also my clown knifes. I moved them from my 300 gallon to my 700 and both would jump from one end of a 6 foot tank to the other and hit the wall on the fly. Imagine a 24" fish whizzing by your head as you try to net it. You almost had to make it jump and try to swipe it out of mid air because that was the only time that it was helpless. lol