LoD cracks.

[Mwa]

What are they?
Why do they need stiching?
How do you avoid them?

No idea what I'm talking about?
Load a map, then open up the console with ~, you'll see it sayings "Stiched x LoD cracks." at somepoint.

[vcxzet]

cracks caused by reduction in level of detail

[Mwa]

Why does that happen, and how to stop it happening?