Or perhaps, when at an afterparty at somebody's house, he/she decides to cuss someone their hardest down the phone, in front of people he/she has yet to make a "good 1st impression" with....and that you have yet to introduce.
You would expect to use this expression like this:
You: "Sorry about them..he/she is a social hand grenade. Most of the time in public, he/she is fine but it only takes someone to pull the pin and thats it...he/she will go off in your face!"
Friend 2: "Yeah, tonight's been really cool."
You: "Hey, aren't spastics funny!?"