For me it was watching a brother have a heart attack during the meeting
in the auditorium. I was a kid and noticed him shaking and acting
strange. The attendants basically dragged/helped him to the back where
he was eventually picked up by an ambulance. The meeting went on like
nothing at all happened. Apparently this has happened numerous times as
others in my group had similar experiences. Normal humans don't react
that way. The show must go on in the KH though.
I remember the same thing happening in my KH when I was a kid.
The strangest thing I can remember when I was about 8 years old was an elders wife come into the KH in the middle of the Watchtower study with a black eye and her arm in a cast shouting and raving about her husband beating her up.
She was dragged out by a gang of brothers and there were gasps from those in attendance. After the meeting we were told she's turned apostate and makes up lies about her husband - a dear elder in our hall. It was still talked about for years after.
Turns out he was indeed a wife beater who'd been beating the shit out of her for years and things came to a head when their adult daughter died. She told the elders, who did nothing. My dad witnessed with his own eyes him slap her across the face. He told the elders, they removed my dads "privileges" and counseled him on gossip.
From then on, although my dad always believed the religion, he had no faith in the people in it.