If you look carefully you will see the very tops of the mountains "grow and sharpen" or "shrink and smoothen" when they pass a certain distance. I saw that same behavior in an old game Starsiege Tribes( from 1998 ). Now I know why it happens.
Popping: it "naturally" happens with all dynamic LOD systems (back in '98 it would have been Real-time Adaptive Optimized Meshes). It happens when something changes from one detail level to another (and hence more details "pops" into view). If you spend more time on your renderer you will generally morph those vertices between the lower detail position and the higher over a very small amount of time. This means that the pop happens over a longer duration and humans are bad at noticing stuff like that.
Incorrect Heuristics: a heuristic is used to determine how far away from the camera you need to be in order to not see the difference between four triangles and two (this has a lot to do with Nyquist-associated theorems) because "the difference is less than a pixel." If you get it wrong you could present too little detail for a pixel and change the apparent shape of the terrain (obviously intentionally presenting too little detail increases framerate).
