When you load a single biome world with a nether biome, the sky is normal except for the area near the horizon, which has a reddish fog. In the basalt deltas, underwater fog is gray, and particles appear black underwater. Occasionally, lava appears next to water without forming obsidian/cobblestone/stone. Also, you cannot see the bottom half of half submerged magma cubes from the top, and vice versa. In the end, the day sky is black, and the horizon area is purple. Additionally, when you load as "The End" biome, obsidian pillars form with their end crystals, but the End Portal Fountain and the Ender Dragon are nowhere to be found (try /tp @e[type=minecraft:ender_dragon] ~ ~ ~). If you load as "The Void", there isn't actually a void, just a bunch of stone and water. Sometimes, is seems like there is a perfectly cut out rectangle with a different hill on the inside (it's hard to explain, see last screenshot. I don't know if this happens often or was just a glitch with the seed)

How to reproduce:

1. Load a world as single biome with the biome being any nether/end biome or "The Void".
2. Look around; some bugs might be harder to spot.