Their own literature condemns them - no matter how hard they try to wriggle out of it afterwards: One of many examples being The Watchtower of August 15 1968, which even ran an article entitled "Why are you looking forward to 1975?"
I remember the excitement of that article too. Interestingly, when I mentioned this to my son-in-law about five years ago, he told me I was wrong. He said his family came into the "truth" about 1975, and they never heard anything about Armageddon coming in 1975. I believe what Witness 007 said above is true, that they started to downplay it earlier that year.
However, it was this conversation that ended my relationship with son-in-law, daughter, my father and several other relatives, and it branded me as an apostate.