This is a nonmanifold edge:
Draw the larger box and the two smaller boxes embedded into the top.
BooleanDifference one small box from the larger.
Then the second small box to get a non-manifold edge.
If you try to Boolean both small boxes from the larger at the same time, V6 will bark at you and place a annotation Dot with an exclamation mark in it.
Select the Dot and it’s secondary text will tell you the intersection curve ends at a non-manifold edge, and fails.