Put differently, it’s understood that all software changes incur some risk, and it’s critical to be able to manage this risk on your own terms. Taking that risk in development is good because by definition that’s when you’re incorporating and testing software changes. On the other hand, if you’re shipping production software, you probably don’t want to take this risk when cutting a release candidate (i.e. build time) or when you actually ship (i.e. deploy time) because you want to validate whatever you ship.
You can address a simple case of this problem by only depending on specific versions of packages, allowing no semver flexibility at all, but this falls apart when you depend on packages that don’t also adopt the same principle. Many of us at Joyent started wondering: can we generalize this approach?