My SIL has called off 4 weddings. The first one was 1 week before the wedding. I had just met DH so I didn't really know his sister very well, but no one seemed that surprised. They all really liked the guy, though.
#2 was a couple years later and they decided to elope. They were going to fly somewhere to do it, I can't remember where, but the day they were supposed to leave she just disappeared. No one heard from her for 3 days, not even her 5 year old son, who was staying with my MIL. She never did talk to the fiancé, she just moved all of her stuff and her son's out of their apartment. The fiancé called everyone in the family, completely distraught, because he had no idea what happened. No one had the heart to tell him that after she moved out of their home, she and her son moved in with another guy.
#3 was the worst of them all. She was engaged to a really great guy, very sweet and kind, and he was wonderful with her son. No one really believed she would follow through, but we liked the guy so much we were all hoping this was it. 2 months before the wedding, she abruptly left the fiancé, kind of like what happened with #2. She had learned she was pregnant and it wasn't the fiancé's baby. She moved out of the house she shared with the fiancé and straight into the baby's father's home, with plans to marry. Her son was a mess. He truly loved the fiancé and was devastated when they broke up. Plus, the new guy was a jerk and did not teat her son very well. A few weeks later, she miscarried. She moved out the next day and tried to go back to the fiancé. He took her back, but a few months later, he called it quits. He tried to do it for her son, but she had hurt him too much. We had become very good friends with the fiancé and we felt terrible for him.
When she told us about #4, everyone was like "OK, whatever." She got really upset because no one seemed happy for her. Since my MIL refused to give her any money, she and the fiancé decided to have a BBQ in their backyard, with the wedding beforehand. At this time, she was living in a different state than the rest of the family, except for us. We lived a few minutes away from her. A few days before the wedding, she moved out when her fiancé was at work, hired a moving truck, and moved back to her home state. She never said a word to the fiancé, or us for that matter. She didn't even say good-bye to us or our kids. And she didn't give her son a chance to say good-bye to his friends. She picked him up from school early and left. We never really did find out what happened. But we weren't surprised.