Feeling sad here, my garden camera caught a great tit flying up and clinging to our nesting box - home to a family of blue tits - it then proceeded to tip its head inside and grab one of the chicks before flying off with it. I’ve seen woodpeckers attacking a nesting box but never a great tit!
Although it's quite hard to believe, it's long been known (many decades) that great tits do prey on small birds sometimes. That incls long tailed tits too.