BuddyPress.org

OK, here is the exact problem. The correct permalink to this page is as follows:

https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/this-is-just-a-test-post-to-determine-if-the-awesome-new-activity-permalinks-are-broken-by-long-topic-titles-that-is-my-hypothesis-and-if-the-permalinks-are-broken-on-this-one-then-i-shall-file-a-t/

The permalink on https://buddypress.org/community/activity/ is truncated:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/this-is-just-a-test-post-to-determine-if-the-awesome-new-activity-permal

That points to the “BuddyPress now has it’s own bbPress forums” post, which I can only assume is the first post in the database.

On a related note, malformed/nonexistent topic slugs should trigger a 404, when instead, they are also defaulting to that alleged first post:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/omg-freaky-weird-nonexistent-topic-slug-that-is-not-real

Update: This is now on trac at https://trac.buddypress.org/ticket/2349