I can't remember where I saw/read the statistic, but it was a VERY high percentage (over 75%) of knife attacks involved a heat of the moment type thing
I think that's why the U.K. physicians want knife tips dulled on large kitchen cutting knives--so many are domestic assault or otherwise unplanned attacks.
I was thinking of the street mugger situation--he's likely learned a thing or two from a friend/gangbanger/cellmate, andif that's a thing you don't know how to handle, it's one thing too many.