From @topisani on February 20, 2016 18:35

This ocacionally happens, i have no idea why. Is it in 1.7 as well?

Copied from original issue: Team-RTG/1.8.9-Realistic-Terrain-Generation#8

Cross-version duplicate of #456

Closing as this it's quite rare and trying to improve it in 1.7.10/1.8.9 isn't worth the amount of time it would take.