Two parts of a UI unexpectedly change the same object. How would you explain and fix the shared-reference mutation?

I would also check whether a cache, selector, reducer, or optimistic update relies on reference equality. In-place mutation can make one consumer rerender unexpectedly while another misses the change because its top-level reference stayed stable. Development-time freezing or immutable-state checks can expose writes, but the durable fix is clear ownership and tested update semantics, especially around rollback and concurrent optimistic changes. I add a regression test that proves the previous snapshot remains unchanged and that a failed optimistic operation restores the intended state.