The Bug

Moving along the Y axis causes the biome to change in the F3 menu. Tested in bodies of water.

To Reproduce

1. Find an ocean or river on a multiplayer server.
2. Go to the border of the land and body of water.
3. Sink and rise, and the reported biome will change.
4. I've been unable to recreate in singleplayer thus far.

Additional

I first noticed this when expanding the edges of a mushroom island in a cove area. I filled in the area of water that was being reported as "Mushroom Fields" or "Mushroom Fields Shore". When I came back later, hostile mobs had spawned in the filled-in cove.