|1.||Post-facto spoiler alert|
When you accidentally spoil something for someone and then attempt to make it up by yelling out "Spoiler alert!"
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.
Kris: "So I'm reading this Harry Potter book and so far I don't like that Snape fellow."
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*