Version lock is when we rely on absolute versions, both FacebookApi and TwitterApi rely on v1.0.0 of JsonLibrary. JsonLibrary v1.1.0 comes out and FacebookApi decides to upgrade. If our dependency management relies on exact versions we cannot upgrade our application to use FacebookApi because TwitterApi references v1.0.0. The only way we can upgrade is if TwitterApi also upgrades to v1.1.0 of JsonLibrary.