That's the dumb thing! I kept all my bettas in bowls for the longest time, then started getting afraid of them jumping, so I put them in the 10 gallon divided... covered up ALL the holes, everything...
He was out of the tank for 10 minutes tops, but covered! And for all of 2 minutes while I was replacing the dividers, he jumped... Just one of those thngs, I guess...
At least he had a good life. He was a full-grown guy when I got him, and he was with me for over a year. Never an illness or anything. He'll be missed.