I used mine that was expired by one day, a couple weeks ago. The clerk looked at me and sighed. Then said, "It's expired. I'll take it, but don't tell anyone. I could lose my job for this."
Seemed overdramatic, especially since I've heard so many other people say they've used expired cash. And I don't know why a clerk would risk her job for a stranger, if that truly was the policy. Seemed like she just wanted to let me know she was doing me a favor. But I was glad to get the $10!