Amish sermons could often be peppered with dire warnings of what happens to those who choose to leave the Amish, or awful things that happened to rebellious youth. Where these horror stories originated I have no idea, but they did what the preachers wanted them to do, strike fear into anyone that was listening.
There was a family that decided to leave the Amish, as the woman was busy packing boxes she happened to look out the window and noticed something odd in the graveyard across the road. She dropped everything and went to investigate, as she got closer she could see there were flames shooting out of a grave and a woman writhing in torment. She noticed it was the grave of a woman who had left the Amish, and now she was suffering for all eternity. After gazing horrified at the scene she went back to her house and unpacked all the boxes. She didn't want to have the same fate as the woman who no longer had a chance to repent.
Another story, and judging from how often I heard it, apparently a favorite to use as a warning to not allow pride or worldly influence into your life.
There was a rebellious young girl who decided she wants to not live according to the guidelines of the Amish church and went and painted her fingernails bright red. Her mother tearfully pleaded with her to remove the paint, but she flippantly disregarded her pleas. The next day she wanted to do the laundry and as she was filling the motor on the washing machine with gas it caught on fire and badly burned her hands and for the rest of her life she had very ugly hands all because she had chosen to allow pride in her life and painted her nails, and God had to punish her.
A man had left the Amish and allowed himself to become baptized again, but after a few years his eyes were opened and he wanted to return to the Amish. On Sundays when it was time for him to join the preachers in their meetings while the rest of the church was singing he would freeze on the bench unable to move a muscle. He had committed an unforgivable sin allowing himself to be baptized again, and now God kept him from ever being able to repent and join the Amish church again.
One of the weirdest stories involved a man growing to a stump he was sitting on and when they tried to cut the stump it bled. For the rest of his days he had to sit there. They built a shanty over him to protect him from the weather, and his wife used to bring him his food to eat. I don't remember what his sin had been, but I do remember being afraid of tree stumps for quite a while.