ABC had a burgeoning soap opera empire on its hands with General Hospital, One Life to Live and All My Children. One would think that they'd treat their back catalog of episodes carefully.
However, the network apparently thought that saving money in a foolish way was a better option. Instead of preserving their tapes, ABC erased and reused them. Up until 1978. The first ten years of One Life to Live's existence has literally disappeared.
Luckily, ABC is now owned by The Walt Disney Company, which has shown more care in preserving its history.