Yea, I was helping a neighbour do a major system update late one evening.  "Time remaining: 30 minutes".  Okay, I'm going home; I'll come back tomorrow and finish.  Two minutes later, "Time remaining: 10 minutes".  Well, I may as well wait then and be there to do whatever comes next.  Until it changed to 15 minutes a few minutes later.  Then from 8 minutes remaining it was suddenly done!