@sam replied to @codinghorror here. But @codinghorror didn’t post anything in that topic and if you click on his name nothing happens.
Unless he did post, but the post was deleted.
Anyway, odd behaviour I’d say.
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It is a subtle “delete” bug.
If you delete the post then replies to the post still keep the breadcrumb.
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I don’t see this as a bug, the metadata is still valid, and the post may be undeleted at some time too.
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If it’s intended to work that way, then it’s not a bug indeed. But it’s a bit confusing. @Sam’s post says it’s a reply to your post, but noone can see it.
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I think the correct behavior would be to show an error message when the user tries to expand this post, stating that he doesn’t have the required permissions / the post was deleted 
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You can test a similar behavior by quoting a section of a post, deleting the post you quoted from, then trying to expand the quote.
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Could we at least throw an error message of some kind here, explaining what happened?
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I beleive trying to expand a quote of a post that is deleted works OK now. As I recall it shows the trash can icon when expanded. Note that I am speaking about actual quotes though 