I had a buttonhole tear when I had DD. It was too high up to be repaired direct, so they had to cut me and create a 4th degree tear to do the repair.
I'm expecting DC2 now and unsure as to whether I should give birth vaginally or opt for a CS. Not many people seem to have buttonhole tears, and I don't know why it happened, or if the scarring means I'm likely to tear again. I'd love to hear from anyone else who has experienced this.