Twitchell's theory of Eckankar includes the principle of "Soul Travel" which holds that one can experience the perspective and knowledge of one's own eternal soul. The soul, according to Twitchell, as full awareness of one's entire existence, unhindered by the limits of space and time. If people are able to access this knowledge this would include the precise knowledge of one's own death. Since Twitchell claimed he had access to this knowledge he should have been able to predict the exact date of his death. Since he predicted incorrectly, he must not have been able to access the knowledge he claimed that he could. Therefore, one cannot be certain that he wasn't also incorrect about his other teachings.
There is no such thing as magic, magic is simply not real. So when someone suggests that they can see the future, they are a liar. The fact that this guy said that he would die on such and such a day and then did not just further proves this fact.
I know it's not a very debate like thing to say, but: Um, DUH. No one is psychic, no one can predict their own death. No one should rely on the "teachings" of anyone else when it comes to spirituality or enlightenment. Otherwise, you simply end up looking like a fool.
Yes, Paul Twitchell's failure to accurately predict the date of his own death should throw doubt on the teachings of Eckankar, because if Paul Twitchell were truly ordained, he would be able to do miracles. His prophecies would come true. But because his prophecies, especially his important ones, have not come true, he is probably just making up his so-called religion, hoping that others will go along with it.
I do not believe Paul Twitchell's failure to accurately predict the date of his own death throws doubt to the teachings of Eckankar. Doubts in the Eckankar way of believing should come from the fact that he may have plagiarized a large deal of his books and belief systems from other sources.