The summary on the revision was just `Depends on D80357`. This existed because the revision was submitted with `arc` instead of `moz-phab`. The stack had been manually reordered via the Phabricator UI positioning D80357 as D80358's _child_ rather than parent. Editing the revision caused Phabricator to parse the summary and try to reattach D80357 as the parent, resulting in a dependency loop. Looks like clearing the summary while updating the title was the fix. We should file an issue upstream to make that error message clearer: it should state clearly that it's the result of a dependency loop highlighting the relevant revisions.
Bug 1647570 Comment 1 Edit History
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The summary on the revision was just `Depends on D80357`. The stack had been manually reordered via the Phabricator UI positioning D80357 as D80358's _child_ rather than parent. Editing the revision caused Phabricator to parse the summary and try to reattach D80357 as the parent, resulting in a dependency loop. Looks like clearing the summary while updating the title was the fix. We should file an issue upstream to make that error message clearer: it should state clearly that it's the result of a dependency loop highlighting the relevant revisions.