For me, the first flash of light was at the Watchtower study in 1995, I think. Being only a mediocre witness, I hadn't studied, so was unprepared for the change in the "this generation" interpretation.
I sat reading the paragraph over and over, and wondering if I was understanding this correctly. I was shocked! As I looked around the room, every smiling nodding head seemed less moved by the change than if they just switched our bookstudy from Tuesday to Monday.
In one Sunday lesson, God's undeniable truth that was taught for decades was turned upside down, and all of a sudden was untrue. Now this other "truth" takes its place, and everyone in the room seemed to accept it without question.
I didn't leave until a few years later, but that one meeting shook me up, and my loyalty was no longer unquestioning. Of course, I thought I was the only one who had a problem with the change.
A few years later, when I fell in love with my "worldly" husband, I had to make a choice. He was okay with me being a witness, except for the blood issue, especially if we had any children. If I was going to risk being disfellowshipped for marrying out of "the truth" and possibly risking future children's lives by the no-blood policy, I had to confirm to myself that I was making the correct choice by staying in the organisation, and to me the way to do that was to verify the authority of the WTS. That was based on being chosen by Christ in 1918, based on Christ's presence in 1914, based on Jerusalem's destruction in 607 BCE. So 607 was where I started and I spent an entire year trying to find some way to validate 607, which just couldn't be done.
I had to admit the WTS was self-appointed instead of God-appointed, started grieving and then healing.
P.S. I'm happily married to my soulmate, have three beautiful children, and no "no blood cards" in sight.