40K: No Retreat! combat assault results loophole?
I can imagine a few situations that would trigger this rule. 

There are a few special rules that give a unit a +1 to combat resolution.  So if the fearless unit whiffs and gets no kills, they can still "loose".  

Another situation is if there are two units involved in an assault with a fearless unit, and the one that can hurt the fearless unit is wiped out.

Finally, if the unit that assaulted was able to hurt the fearless unit only by means of Furious Charge's +1 to ST.

There are probably a few other situations.  It's defiantly an edge case and I've never seen it come up, and I've been involved in running numerous large(30-60 person) 40K tournaments.