Yes, once. I immediately regretted it, even though it was a relatively trivial thing. I don't think I'll ever do such a thing again.
No. Im trustworthy.
No.... if they've taken you in their confidence then you should respect that.
Under what circumstances?
That's not for you to get involved with.... you should speak to the friend who's confided in you and explain to them what people are saying. It's for them to decide, not you.