What's the biggest cock-up by HR that you've witnessed?

My friend was telling me about how the latest drama in their org is that someone was promoted to a managerial role. Big announcement email sent out to all staff, people in the office congratulating that person etc. A few hours later, HR announce that they've "suddenly" realised that someone else would be better suited for the role. Cue embarrassment.

Now I'm curious if there are any other stories like this.