Abigail can be pushed out into the ocean with no way of getting back. It was evening, and she was surrounded by four or five pigmen that grouped around her due to light, and they ended up pushing her out over the ocean. I had to get her over to the edge and kill her so we could respawn her. I'm assuming that Klei knows that things can fall in the water regardless (say one kills a Gobbler next to the ocean, drumsticks fall in ocean).
Get a bunch of pig men, summon Abigail, and be near ocean. If they have her pinned, walk away so that the shortest path for Abigail to join you is to go across the ocean.