Abstract: The rules that govern the creation of an estate plan are in flux. Courts once demanded strict adherence to the Wills Act. Yet, this legacy of hyper-vigilance is waning, as the Uniform Probate Code, the Restatement (Third) of Property, and ten states have adopted the harmless error rule. Meanwhile, trusts, which need not comply with the Wills Act, have eclipsed wills as the dominant method of posthumous wealth transmission. This Article explores three budding topics that threaten to further complicate this area. First, there are anecdotal accounts of decedents trying to make electronic wills. In both strict compliance and harmless error jurisdictions, e-wills raise thorny issues about the meaning of “signed” and “writing” in the Wills Act, and when, if ever, courts should be able to overlook violations of the statute. Second, despite the received wisdom that trusts are less formal than wills, a rising number of settlors are failing to observe the arcane principles that govern the transfer of property into a trust. Third, most state legislatures have adopted or are currently considering statutes that give fiduciaries access to the contents of a decedent’s email, text messaging, and social media accounts. But the precise steps necessary to convey these cutting-edge forms of property after death is unclear. This Article tries to help courts and policymakers regulate these matters by offering a fresh perspective on the purpose of mechanical, bright-line principles in the realm of estate planning. As conventionally framed, this debate revolves around what the Article calls the “intent paradigm”: the idea that execution doctrines should be gauged primarily by whether they facilitate or frustrate the wishes of individual decedents. Conversely, this Article explores a different virtue of formalism: its ability to prevent decedents from imposing spillover costs. This Article demonstrates how some unyielding principles limit the burden on courts, survivors, trustees, the trustee’s creditors, purchasers of trust property, and other third parties. It then explains how recognizing this anti-externality function can pay dividends in wills law, trust law, and emerging niches such as the inheritability of digital assets.