Hahaha, that is so true and so annoying. I am perpetually scared of sending the wrong thing in the wrong group. It gets even worse if the groups have similar names. If only there was a way to filter on excluded members...
And if your group has a very large number of subgroups, it's a sign group health is deteriorating. Side channels are healthy, but if they become the main mode of conversation, congrats, you're starting to develop cliques.