Final nail? Don't know yet, we're still attending.
I'm going to define the first nail as what was the thing that gave you the mental green light to REALLY start looking at the WT with the blinders off.
First nail for me was giving a talk on the Service Meeting a few years ago on how to treat DFed relatives. I had personally shunned my older brother for about 20 years. I mentioned this in the talk. Did a very good job and was commended by many. One in particular said she was going to take the counsel more seriously. (Shudder) That did it. What the hell kind of a human being was I?
First nail for my wife was an speaker going off about not even taking a glance at anything that smacks of apostate thinking. It seemed VERY extreme to her. When we got home I used an "illustration" I learned here on JWN.
Imagine that a large envelope arrives on our doorstep with a note on the outside explaining the contents. Inside the envelope is irrefutable evidence that I am cheating on my wife. I tell her that I am NOT cheating on her and DON'T YOU DARE OPEN IT! DON'T EVEN TAKE A LOOK! If you love me and trust me, YOU WON'T EVEN LOOK!
My wife told me last night that this was what finally gave her the confidence to look inside Pandora's box.
Methinks Watchtower doth protest too much.