I have unwittingly. I was having an affair with a married friend, and one early afternoon when I was eating her pussy she told me to be careful. I realised she had fucked her husband that morning and his cum seemed to be tricking out. I thought I would hate it but it made me so hard with the thought that I had one over on him, and I loved the the taste and smell of stale sex. It hasn't happened again, but it was such an exciting experience.