My initial reaction to somebody saying that to me would be, "have you lost your mind?"
>>That's precisely what I thought when I came across the word for the first time, but unfortunately I met someone with a lost mind a couple of days back. There are people who do that! There's no dearth of people who have lost their minds - which is obvious from the fact that there's a word for this kind of (can irritating be innocuous?) behavior :)