Both Mants and Weevoles spawned in the pond and were unable to get onto land, instead traveling across the pond like purely aquatic mobs or players on boats.
The Mant Hills in both pictures are up against the pond so that the Mant's spawning on the pond-side of the hill would spawn over the pond border.
In the Weevole's case, several Tall Grass tufts actually spawned in the pond, and were unreachable when I tried to dig them from the shore.
Steps to Reproduce
Mobs spawners spawn in too close to the water's edge.
Mobs spawners spawn in too close to the water's edge.