Ashley's/Kaidan's rejection of you on Horizon.If you send a traumatized Veetor to Cerberus instead of letting Tali take him, he'll show up later in her loyalty mission, even more of a nervous wreck and unable to travel without a doctor.Even knowing it gets replaced by the SR-2 doesn't do much to take the edge off when it happens. The first scene, where you see your beloved Normandy go down.
