When a migration is rejected after a post is migrated, the post migrates back to the original site.
Normally with migrations you can see the revision history of the migrated post (to give attribution to the editors under CC-BY-SA), but when a post is migrated away and subsequently "returned", one can't see the revision history on the site it was originally migrated to; the page 404's.
I can see this behaviour with this question, which was migrated from Electrical Engineering Stack Exchange to Stack Overflow, then back again, where the links in the top notice and revisions page 404 for me: