Right there. Shot the kid and got the "new" ending. Did the same thing: Loaded up a old saved game, worked my way to the ending, accidently shot the kid and then sat there and said "ugh...crap", then went back and got the ending i was going for.
Despite having to go back, i was cracking up about it, complete surprise. Apparently they also ninja'd in some other stuff that people are still tracking down. One is about the two engineers from ME2(Daniels and Donnelly), and another has something to do with a potential upcoming single player DLC about the "Leviathan of Dis"(hints about it go back to ME1).