Lincoln Craven-Brightman

This is not a bug. The water was drained, and then began to flow again. Source blocks were created in the normal way water source blocks are, and the water eventually returned to normal. To avoid this mistake in the future, section off areas that you plan to drain so that water cannot create new source blocks in the area that you just emptied.

This happened to me once, occurred after a strange chunk issue. He had not appeared before that point and did not appear after I rejoined the world.