They guy/gal who decides to slow their aircraft without saying anything to ATC. This might be fine in Fargo (nothing against Fargo, loved the movie) but when 20+ aircraft are behind you, and spacing is predicated on you not slowing cause "I heard you tell the last guy short approach so I was setting up for one" or any other excuse, causes a massive chain reaction. I've seen this day in and day out lately and some of the chain reactions I've seen just this week were downright dangerous.

Just ask