Thanks for those pointers. Now that I know it, I see it everywhere 
To me this feels like a big enough deal I'd like dplyr to chattily warn me about it, the way that it does when I use explicit joining. I think it should return red text after the summarize that says something like, 'summary complete, group_by reduced to 'fieldA', 'fieldB', etc.
I may bring that up on the dplyr github.