How Salesforce's Dual Security Runtimes Silently Break LWC Components in Production

A Salesforce Lightning Web Component developer encountered a silent production failure affecting only some customers after the component passed all pre-release testing and QA. The root cause was a collision between two coexisting security runtimes on the Salesforce platform: the older Locker Service and the newer Lightning Web Security. Because individual orgs control which runtime is active based on settings and API version, the same packaged component can behave differently across customer environments without throwing any explicit error. Lightning Web Security enforces Content Security Policy more strictly than Locker Service, meaning JavaScript patterns such as dynamic evaluation or certain DOM manipulations that work under one runtime can silently fail under the other. The developer notes that this dual-runtime reality is rarely documented in practical terms, leaving ISV developers vulnerable to hard-to-diagnose production issues.