Short Answer: Book of Abraham
I used to be an amateur Mormon apologist and I believed strongly in the witness of the Spirit. It was my perception that I had more experiences with the Spirit than most members. On countless, various occasions I felt the warmth of the Spirit in my chest, peace in my heart and mind, overwhelming love, what I can only describe as a brightness and clarity in thought, a sure confidence, a connection with God that was so real that it approached tangibility, a knowledge that God was my friend and father, excited joy, a passion for helping the Lord prepare for his Second Coming, a sense of the immensity and beauty of God's plan, heart-aching in prayers, crying out of amazement and gratitude, I gave beautiful priesthood blessings that meant special things to the recepient that I was not aware of, people recovered from their illnesses and sometimes were given instant pain relief and peace, I felt my ancestors prompt and cheer me in my family history and temple work, I had what I believed to be an angelic visitation when I was 9 years old, I sometimes sensed that certain things would happen before they did, I miraculously got unexpected scholarship checks when I needed them, etc. I knew and loved the scriptures and tons about church history. I read "Joseph Smith Papers" (Joseph's diary) because I so wanted to know the man and be able to receive the revelations he received. I even tried to find my own peepstones so that I could learn how to focus more intently on the still small voice.
In my intense interest to soak up all things related to the Church and Kingdom of God, I read a lot of original works and got exposed to many issues that cause members to question Mormonism. Due to all of my spiritual witnesses and insight I felt I had gained, I always dug for the explanation, the way to understand why certain things were done the way they were (i.e., how polyandry was right and approved by God, how God used Joseph's interest in peepstones to train him to listen to the Spirit, etc). And with every issue I came across, there was always some way to interpret or explain the data in a way that was favorable to Joseph. That was until I came to the Book of Abraham. After 6 months of studying all apologetic and critical and original sources, I had to face the fact that there was no workable hypothesis that sustained Joseph that did not ignore data. The Book of Abraham was not only not a translation, in all likelihood it was not a revelation, and the evidence points to it being a knowing fraud by Joseph.
Well, that shook me to the core. This wasn't some "prophets are imperfect" situation. This was the chosen leader of this dispensation while acting in the office of prophet, seer, revelator, and spokesman for God, at a time when he is supposed to be as in tune with God as ever, purposefully fabricates false scripture that has been cannonized and accepted by the whole Church. I had previously had a spiritual witness that the Book of Abraham was what Joseph claimed it to be. This witness met all the requirements that we have been taught to be a true revelation and not some feeling I had created and mistook for the Spirit. Presumably there would be many members, even Presidents of the Church that had had the same witness, and yet I now knew that we had all been duped for the BoA is not anything close to what it is purported to be. This is not to say that it does not contain passages that can be meaningful and give comfort to people, but that does not make it scripture.
So, at minimum, I had to conclude that the experiences that I had previously called the Spirit were unreliable as a method for discovering truth, not only for me, but apparently for modern day prophets as well. This is not to say that those feelings are wrong every time, but I could not tell when they were right and when they were wrong (and apparently neither could anyone else), so they should not be relied on. I thought back on the many personal experiences I had had when I interpreted the Spirit as telling me something and I was wrong. For example, on my mission I was certain through a powerful spiritual witness that when I blessed a convert of mine she would be healed of her endometriosis; or another time when I just knew by the Spirit that I would marry a particular girl. Neither thing happened. These things happen all the time in the church but we right them off as lack of faith or hormones or whatever. What if we did not find some excuse for them but stare them square in the face?
Those were big struggles for me at the time; I doubted myself after they happened and concluded that I was out of tune or did something unworthy to make the prompting not come true. But, now when I reconsidered those experiences, maybe I had not done anything wrong or was out of tune, maybe this unreliable Spirit never speaks to anyone and it is all in our heads. So, I tried out that theory. What if the Spirit isn't real? Can I account for everything in a purely naturalistic world? I found that I could. I have never come across any hard physical evidence that does not have a potential natural explanation. Now, is that explanation the right one, sometimes we may never know because we cannot cross-examine or replicate in a controlled environment. I have found tons of problems with interpreting reality with a supernatural component. I find the naturalistic view much safer and will stick with it, until compelled by evidence that naturalism is incapable of accounting for all observations. I don't want to be fooled again or taken advantage of by opportunistic charlatans that claim access to supernatural worlds.