Issue #20444: Human Error Quest - Cannot be started
As documented here (http://fallout.wikia.com/wiki/Human_Error), Honest Dan disappears after the main quest and accordingly, the Human Error quest cannot be started.
I'm not sure whether this behaviour was intended by Bethesda, but it seems like a bug to me.
I've logged a support request with Bethesda, but I'm not holding my breath for a useful response.
Got response from Bethesda. Something along the lines of "given the issue has been widely reported, it's a safe bet the dev team is looking at it".
Certainly hope so, because I can't find anywhere he's referenced by something that could disable him in that way.
There are designer notes somewhere on that quest saying that this quest should become unavailable after the Institute has been destroyed.
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