Except when it's not. The only way to find out is to open the box. Er, I mean click the link.

Others may be people that somehow 'peeked in that box' enough times they didn't want to do it again and their local memory has stored the URL and associated it with a negative outcome. URL shortener's totally destroy that learnt ability.

There's kind of a reason people came up with the term "risky click of the day" and now there's 'NSFL' (Not Safe For Life), which is extremely worrying.

Anyway, I got upvoted which means it still worked, and I got to legitimitely rickroll someone on HN and they appreciated the lesson. Woohoo.

One of the problems with the software engineers solution to these kinds of problems is that create a plugin that checks whether the url resolves to the rickroll video or not. The problem is definitely solved, but now the computer/browser is slower for everyday use. Also the problem is still easily recreatable at a different level of abstraction. (e.g. aforementioned URL shorteners or copying the rickroll video to another location) <-- this is the really hard problem to solve and we haven't even used encryption yet.

When it reaches the level of the politicians 'protect the children' where its about votes and appearing to do the right thing but none of the people understand the implementation problems and we discuss the absurdity of it on HN.

You do however get to be technically correct. It's not a proof. It's more like circumstantial evidence in experiential form.