When a zero-day vulnerability is discovered in mission-critical systems that require high availability, immediate patching is often not possible due to lack of available patches or the risk of disrupting critical operations. In such cases, the best practice is to implement compensating controls (such as increased monitoring, access controls, network segmentation, or web application firewalls) to mitigate risk until a patch or permanent solution can be safely applied.
Reference:
CompTIA Security+ SY0-701 Official Study Guide, Domain 2.4: "For zero-day vulnerabilities in critical systems, compensating controls and heightened monitoring are often necessary to maintain availability and security until an official patch is available." Exam Objectives 2.4: "Given a scenario, implement secure system design."