Placing water will sometimes cause new source blocks to generate out of nowhere.

Steps to reproduce:

1. I created a world. (Seed: 2253821182612647832)

2. Go to X: 284, Y: 65, Z: 344

3. Place a water source block and let it spread.

4. Three new source blocks will be created.

Note: It might take a few tries. If it doesn't work, delete the source block and try again. Takes 1-3 tries.