In his write-up of Pro Tour BTG in Valencia, pro player Gaudenis Vidugiris described this game, the last one he played to get to the top 16 (emphasis added):
Round 14 I battled against RWU Twin. Both games I got active Ascensions reasonably quickly and killed him. Though in the second game he had a Grafdigger's Cage on turn one which slowed me down enough that I couldn’t kill him for a while and even gave him a draw phase where a Splinter Twin would have been lethal. I actually got two active [Pyromancer Ascensions] rather quickly, but couldn't find a blue card to copy for a turn or two. Eventually, I found one which dug me into an Echoing Truth and after I explained to my opponent why his overloaded Counterflux couldn't counter all of the Echoing Truth copies he conceded.
What's the rules trick here? How come an overloaded Counterflux can't counter several copies of Echoing Truth?