From the page for the Heroic Blue Screen of Death:
In Torchwood: Children of Earth, Captain Jack Harkness shuts down twice. The first time is after his defiance of the monster of the week, the 456, results in the release of a virus that kills everyone in the building, including Jack’s lover Ianto Jones. Later, he goes AWOL from England, and ultimately Earth, after he kills his grandson in order to save the millions of children who would otherwise have been taken by the 456.
From the page for the Tear Jerker:
...The moment when Jack realizes the consequences of the 456's response to his ultimatium and blurts, "I take it back!"
...[Frobisher’s choice to kill his family and himself is] Made even worse in retrospect by the fact that the 456 are stopped. Frobisher killed them all for nothing.
...The climax: Jack destroys the 456 by killing his own grandson.
I also wrote something about the major event of Day One for the page titled “It Just Bugs Me”, which is about moments of Fridge Logic, but I think it deserves a separate LJ entry because it may generate some discussion.