Not specifically a betta related story, but definitely fish related. Last year I saved up for a liquid water test kit for my snail, betta, and goldfish tanks. I tested the water in the goldfish tank that night, did a water change, and went to bed. Early that morning, I woke up feeling TERRIBLE. I was jittery, nauseous, dizzy, and in a cold sweat. I googled my symptoms and the only thing I found that wasn't some obscure disease from the Amazon was chemical poisoning!
Turns out I'm very sensitive to the chemical used to test for ammonia. If the tiniest droplet gets on my skin, I get sick. I honestly thought I was going to die! Went to the doctor, he gave me a strong antihistamine and something to help my body process the toxins. It took a few days for me to start feeling better, but I promptly returned that test kit! I can only use strip tests now, which aren't as accurate or reliable, but its a small price to pay to never have to go through that again.