I've had several bettas flare at their reflection til they just couldn't anymore, but they didn't really hurt temselves... It's not good for them but in some cases with glass aquariums and various lighting conditions it can't be helped. I usually find mine in the corner every once in a while usually around the same time of day. On another note, I have used a mirror to get my betta to flare at times when her color seems pale or she seems to be lazy and she livens right up. I read somewhere once that if you use a mirror to make them flare you should pull it away before s/he backs away so as to give them the feeling of being the 'winner' I don't know if that part is true or not, but I do it just in case.