Oneboxing by post_ID can generate a user mention.

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As shown in Onebox Notification Wierdness, you can generate some odd notifications when oneboxing with the format "http://what.thedailywtf.com/p/{postid}". Specifically, if the target post is in a topic that contains a mention, the mentioned user gets a mention notification.

First noticed as a result of this post, as shown here.

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Reported on meta.d

https://meta.discourse.org/t/oneboxed-posts-may-trigger-unexpected-notifications/29007

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And I discovered another interesting detail. Cross instance URLs of the format http://discourse.example.com/p/post_id don't onebox.

https://meta.discourse.org/p/119405