The manuscript peer review process is supposed to be secret, for the most part. The authors are not to know who reviewed their manuscript...this is generally for the protection against potential retaliation and the corresponding expectation of unfettered evaluation.
Yet one often has conversations with ones fellow scientists at conferences where it becomes obvious the other person reviewed your manuscript. Or that you reviewed theirs.
I find, especially lately, that this is *good* for science. You can discuss the issues with the person. Naturally this is only in cases for which the reviewer wasn't a total hater...don't think I've had that conversation yet!