On en:Motu Patlu, an administrator protected the page with semi-protection and then pending changes protection immediately. However, five minutes later, an anonymous editor was able to edit the page. The edit was kept as pending due to the pending changes protection, but the edit still went through the semi-protection.

Attempts to recreate this error have failed. See this discussion. One thought is that the two protections so close to one another caused an issue, but even an examination of the edit shows Edit protection level of the page (article_restrictions_edit) autoconfirmed.