There are so many spoilers in all these posts for already-aired shows that I'm not going to call out specific warnings--except for last season's Alias finale in my own post, so don't click if you don't want to go there.
1. BtVS: The last, terrible battle in "The Wish," where Buffy kills Xander, Oz kills Willow, and the Master kills Buffy.
2. BtVS: Buffy discovers her mother's dead body and calls 911. Everything in that scene.
3. BtVS: The opening scene of "Once More, With Feeling" when I realized that Joss was going to pull it off.
4. BtVS: Oz leaves Willow.
5. BtVS: The first Buffy/Spike kiss.
6. BtVS: The second Buffy/Spike kiss.
7. M*A*S*H: Radar announces that Henry Blake is dead.
8. Alias: Just about every time Sydney cries, including the most awful one: when she finds Danny's body.
9. Alias: "Who are you to come to me and act like a father?"
10. Alias: Will sees Sydney in Paris and realizes she's a spy. (ETA: Okay, actually at that point he has no freaking clue what she's doing there, but it's still priceless. *g*)
11. Alias: Will hugs Jack in gratitude.
12. Alias: Allison stabs Will, crying.
13. Alias: Michael proposes to Sydney.
14. Alias: "My name isn't Michael Vaughn..."
15. QAF: Justin is gay-bashed. Brian in the hospital corridor.
16. QAF: Justin plans a romantic indoor picnic for Brian, who blows it off to go clubbing.
17. QAF: Michael dreams that he's having sex with Ben when the condom breaks, and he says, "Keep going" even though Ben is HIV-positive.
18. QAF: Hunter confesses to Michael and Ben how his mother pimped him out to men.
19: QAF: Lindsay and Melanie appear to be truly ending their relationship.
20: Some show, some heart-crushing moment I thought I'd never forget, which I've forgotten. Ah, brain cells, how I miss you.