A strange bug which involves the Nether, specifically the Nether Portals. Sometimes upon loading into a world while in the Nether, only half of the portal will show. The remaining obsidian/portal blocks are completely gone (no ghost blocks, etc.), simply replaced by normal Netherrack.

What I was doing to achieve this...

• Went into the Nether for the first time
• Generation took a long time so I hit F3 + A to refresh/reload chunks
• The chunks loaded but I was then getting server-style block lag as a result of the generation taking such a long time so I quit out of the world while still in the Nether
• Loaded up the same world. Generation, once again, took quite a while
• For some reason, only half of the Nether Portal appeared.

World Seed - 114093887849088583
World Type - Customised (Ravines = Off, Biome = Roofed Forest, Dungeon Count = 100, Lava Lakes = No, Biome Size = 8, everything else is default)