I deleted a comment, and the forum topic vanished from the forum topic list. There are several older support forum threads on this,
http://drupal.org/node/56558
and this one which describes the exact symptom, but with a different cause (user no longer exists in that case, my case is that the "last" comment no longer exists)
http://drupal.org/node/51415 - #51415: forum topic does not show up in forum index if associated last_comment_uid user no longer exists

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Status: Active » Closed (outdated)

Automatically closed because Drupal 6 is no longer supported. If the issue verifiably applies to later versions, please reopen with details and update the version.