This is very good, at least it will be in a few years. Sorry if it's been posted here before already.
It's in the Watchtower magazine May 15th, 2014, last page: From our archives: They talk about a public speech held in Sao Paolo in 1923 where they also distributed the brochure "Millions who now live will never die" (pictured in the magazine!). A quick calculation: 1923 -> 2014, how many people in the world now are 90 years old or more? Not sure, probably still millions, but not many. So in a few years there is definite proof about the fact, that at least once they printed something wrong. And you don't need to look for it in the internet, apostate literature or decades old watchtower magazines. You can show them openly with their own latest literature. And then ask the question: "How can you be sure, that everything they print today is true? You can't!"
I definitely planted a seed there (in my wife's head).
PS: I found this in the Spanish version of the watchtower (La Atalaya). The not simplified version! Not sure if it's in the English version.