I just came across this post, whose first link is to here, which is actually broken and goes to a totally wrong post on a wrong site. I noticed that the Linked section on the right contains the correct link to this post. I guess what happened is that the link in the question pointed to the original location of the linked post, but it was subsequently migrated from Astronomy SE to Physics SE, and the link in the Linked section was updated correctly but the link in the post was not. Why is that so? Is there a bug in the SE software?
Educated guess: I think the answer here is that the current Astronomy.SE is the second iteration of that site. The first iteration failed at some stage in the Area51 process and Physics absorbed many of their questions. Later, apparently, a second Astronomy site was begun. I suppose it's possible that some post IDs got re-used.
Note that the correct link, to physics.stackexchange.com/q/26461, is to a question, while the link in the text, which redirects to astronomy.stackexchange.com/a/1211, is to an answer. When the new astronomy site accumulated enough new content that the post ID got re-used, then the link went from "just broken" to being broken in the confusing way that you noticed. However the list of "related posts" is stored in a different table in the database and was apparently migrated correctly when Physics absorbed the first round of Astronomy questions.
The rise and fall and re-rise of Astronomy happened before I joined this community, but there are old posts on Physics Meta about it and probably someone else will chime in with an approximate timeline.