I was trying out the new availability to swap texpacks in-game, and found that apparently, the game does not update the worlds' textures correctly until you place/break a block or load new chunks.

Steps to re-produce:
1. Open a world with a custom texture pack set(I was on a superflat)
2. Pause > Options > Texture Packs > Default
3. Resume the game and observe.
4. Place/break a block or load any chunk.

Edit: The Bedrock floor turned into the bottom half of a bed, the nether quartz's bottom face was pumpkins, but its' side texture was a chiseled quartz block. Also, interestingly, redstone is a bent rail, and a repeater is a flat off-state torch that is emitting red particles.