Wars, typically. Just look at the Balkans, where there are too many boundaries.

Waves break on the beaches.

Rainbows are generated at boundaries.

People get hired at the beginning of a project.  People usually don't quit or get fired until the end.

Paradigm change only happens by PushingTheEnvelope (i.e. the boundary)

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This is something I've been recently been contemplating regarding the WikiWikiWeb itself. Many of the long-running problems I've seen on Wiki come not from those who directly contravene the WikiSocialNorms, but rather from those who flirt with their boundaries. Those who don't understand or agree with the social norms tend to leave quickly, but there's a small set of contributors whose behavior is right on the edge. That tends to lead to friction and Things Happening (at the worst, EditWar''''''s).

In some cases this leads to growth (e.g., the DeleteOnceRestoreOnce principle grew out of one such episode), but often not.

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TheCulture has a single organization, called "Contact", that deals with war, diplomacy, espionage, and other things that happen on its borders.  The name is a clear indication of the significance of boundaries.
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ThingsHappenAtBoundaries, therefore make sure you test the BoundaryConditions.

However, ThingsHappenAtBoundaries, therefore it's a pain in the ass to test the BoundaryConditions.

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In sports, there's a saying like '''the best defense is just near the foul boundary'''