My wife's cousin recently had a baby, and posted a photo on Facebook. My wife commented (something like) "She's so cute I'll have to kidnap her". Facebook locked her account for 24 hours for making a "violent threat". I am sure the mother wasn't threatened by the remark in the slightest. It just added to my wife's anger at Facebook for repeatedly giving her "warnings" over trivial or innocent things.
Not online, in person, but one of the teachers at our son's school sometimes "threatens" to "steal him" from us – her remarks don't worry us, because we know she would never actually do that, it is just a colloquial way of expressing affection.