What are sample gaps during scan conversion? In this tutorial,

It is very often the case that triangles are rendered that share
edges. OpenGL offers a guarantee that, so long as the shared edge
vertex positions are identical, there will be no sample gaps during
scan conversion.

1 What exactly are sample gaps during scan conversion?
2 Under what circumstances, will it happen?
3 Does it happen when the line cross the center of the fragment?
Sample gaps will occur if you don't do things "properly". 

In OpenGL or D3D, assuming a consistent winding order, if you have triangles  ABC and CBD, then when a sample point - that is a test during scan conversion to determine if a point (read pixel) is inside a triangle - lies exactly on the shared edge BC, then that sample will belong to exactly one of those two triangles. 

This avoids both gaps and double filling. The latter would be problematic with, say, translucency or stencil operations.