It will nonetheless piss someone off because either a) you're just rubbing it in that you spoiled the ending to an amazing movie/book/show, or b) they weren't really paying attention before and would have missed the spoiler, but because you had called attention to it, now they are aware they have been spoiled.
Melissa: "Yeah I really hated Snape in the 6th book when he killed Dumbledore but everything worked out in the end."
Kris: "Ye- Wait. Snape KILLS Dumbledore?!"
Melissa: "Er, ah, um, SPOILER ALERT!!!1"
Paul: "Wow, Melissa, way to pull a post-facto spoiler alert."
Kris: "It will never be the same!" *sobs*