Yes, but the problem is that in most of the scenarios that get discussed it's not flirting but one person making advances and not noticing (or caring) that the

Yes, but the problem is that in most of the scenarios that get discussed it's not flirting but one person making advances and not noticing (or caring) that the person is actively feeling uncomfortable. The rule of thumb is sort of in line with telling a risque joke. Is that ever OK in a workplace? Sure, but if there's any doubt whatsoever how it will be received by the audience then you probably shouldn't be doing it.