X-No-Archive
The header line X-No-Archive: yes, which asked Usenet archives to forget a post — and worked right up until someone quoted it.
Once Deja News made old posts searchable from 1995 onwards, a lot of people realised that things written on the assumption they'd expire in a few weeks were now permanent. The fix was a non-standard header the archives promised to honour. Deja News skipped such posts, and Google Groups later removed them after about a week. Anyone whose newsreader couldn't edit headers could put the line at the top of the body instead.
The flaw was obvious: a reply quoting the post carried no such header and was archived in full. Privacy on Usenet came down to your neighbours' quoting habits.