I looked up this topic in a book that I I have which I really recommend: Reasoning from the Scriptures with the Jehovah's Witnesses by Ron Rhodes, I really suggest picking up a copy. I don't have a scanner so bear with me, I'm just gonna do this the old fashioned way.
"Those who have followed the watchtower literature over the years can attest to the fact that the society has consistently changed it's prophecies in order to cover it's many errors. Former Jehovah's Witness William J. Schnell obeserved that "The Watchtower magazine changed our doctrines between 1917 and 1928 no less then 148 times."
"Incredibly, the Watchtower society argues that it should not be critisized for it's past errors regarding prophecy. After all, biblical prophets and apostles made mistakes, and they were not condemned as being false.
"One example Jehovah's Witnesses cite in support of this is the prophet Nathan.When King David wanted to build a house of worship for God,Nathan told David to do as he wished. But later God told Nathan to inform David that was not to be the one who would build the temple. Despite Nathans error, however, Nathan was not condemned by God or by anyone else. Indeed, God continued to use Nathan because he humbly corrected the matter when God made it plain to him.
"Orthodox Christians point out, however, that Nathan's original messege to David about building the house was not claimed by Nathan to be instruction from God (1st Chronicles 17:2).His subsequent messege about not building the house was claimed to be instruction by God(17:3-15). Hence, again, we find the Watchtower society twisting scripture to suit it's own ends.
"Amazingly, despite it's many failed predictions, the Watchtower Society has not learned it's lesson. Even since 1981 it continues to say that Armageddon and the world's end is very near.
"You might remind the Jehovah's Witness that The Watchtower magazine says that when one discovers false prophets, "the people should no longer trust them as safe guides." (May 15, 1930 Watchtower)
* Do you agree with the Watchtower instruction that when one discovers false prophets, then "the people should no longer trust them as safe gudes?"
* Since the Watchtower Society was wrong in 1874, 1914, 1925 and 1975 do you think you should still trust the Society as a "safe guide"?
Another good point to make, is when Jesus supposedly inspected the 'Christian' congregation in 1919, what were the teachings? At that time the idea of Pyramidology was still in use and I believe it was within this time frame after Rutherford was released from prison that he began teaching the 'flock' that 1925 would mark the end. Rather then repenting from making the error regarding 1874 and 1914, they continued on with the same course of action. Even had they repented from that course, Where in the Bible does it teach that that would make everything ok? It doesn't.
Another thing from the book:
* Can you point me to a single example in scripture in which a prophet was issuing a "thus saith the Lord" prophecy direct from the Lord and was subsequently shown to be in error?"
The idea expressed in the book re the apostles was that while they may have had "false notions as human beings" they Never taught that such notions were from God. "No true prophet of God ever made a mistake while uttering a prophecy because he was delivering God's words to humankind, not his own."
This particular book has been very helpful for me. I really suggest that you get a copy. The publisher is Harvest House. Ron Rhodes has written a ton of stuff regarding cults.
Look through 'the best of" threads, you might find some really helpful stuff.
Keep in mind, that if your mom has been a witness for this long, there is so much emotional attachment. Not just friends and family, but the whole idea that she has been duped for this long can make a person feel kinda dumb. Even if you see that she does 'see' that something isn't right, be very patient. Be very loving, reassuring and make it a point to draw close to her. If she slowly realises that things just aren't right with the org, that's a great thing, but as I am sure you already know, she could get very depressed. Older people lose a certain sense of self worth and self esteem as they age. They begin to see how fragile life is, and subsequently, how fragile they are. They start to forget things and start to feel naive. So, anyhoo I wish you so much luck and good for you to have enough love for your mom to reach out to her. Please take care.
P.s. hope i didn't get anything wrong with the book