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When someone sends a picture that is vague enough that is only a spoiler if someone calls it one
“Hey delete that photo! It’s a spoiler!”
“Wait it is? I could have sworn we’ve this before”
“No it’s a spoiler for something later, trust me man this is the Spoiler paradox”
An acknowledgment of a spoiler very much like a spoiler alert except that it's provided after rather than before the spoiler in question. See, for example, the Dinosaur Comics strip for April 15, 2011.
By extension, any would-be warning preceded by the thing it's supposed to warn against.
"So then Snape kills Dumbledore. That's a particularly infamous spoiler." "Nice postfix spoiler alert, you numskull." / A week after the drop deadline, the registrar told me I was taking the wrong classes and wouldn't be able to graduate till next semester. Talk about a postfix spoiler alert! Does he think I'm Merlin or something?