National and international efforts to manage forest biosecurity create tension between opposing sources of ecological and economic irreversibility. Phytosanitary policies designed to protect national borders from biological invasions incur sunk costs deriving from economic and political irreversibilities that incentivizes wait-and-see decision-making. However, the potential for irreversible ecological and economic damages resulting from failed phytosanitary policies argues for precautionary measures, creating sunk benefits while increasing the risk of over-investment in phytosanitary security. Here, we describe the inherent tension between these sources of irreversibility in economic terms, relate these forces to type I and type II errors, and use this framework to review national and international efforts to protect forests from biological invasions. Available historical evidence suggests that wait-and-see phytosanitary decision-making has dominated the adoption of precautionary measures in most regions and that willingness to under-regulate may sometimes be orders of magnitude greater than willingness to over-regulate. Reducing scientific uncertainty about threats to biosecurity may help mitigate the tendency to under-regulate, and phytosanitary security measures with relatively modest sunk costs could help protect forests as scientific learning advances. A fuller accounting of the costs associated with type II errors, particularly regarding the suite of non-market ecosystem services at risk, would help decision-makers better understand the trade-offs between the sunk costs of policies and long-term economic losses to stakeholders.