When using data/mutation libraries, treat error handling as a two-step process: (1) type errors conservatively at the boundary, and (2) implement recovery/UI logic that matches the library’s actual error-state semantics. **1) Use conservative global error typing** - Prefer a global error type of `unknown` (or keep the default) so call sites must explicitly narrow before reading properties. **2) Don’t assume placeholder/optimistic flows preserve error status** - If you migrate from `keepPreviousData` to `placeholderData`, update UI logic: `placeholderData` puts the query into `success` state, even though background refetches could fail. **3) Wire recovery callbacks to the correct inputs** - For optimistic rollback, use the value returned from `onMutate` (as provided to `onError`) rather than relying on older callback signatures. **4) Place try/catch only where the promise can throw** - Some helper calls (e.g., prefetch) return a `Promise` that you can await but “will never throw”, so don’t wrap them in try/catch as if they were failing operations. Example (TanStack Query placeholder + conservative error typing): ```ts import { useQuery } from '@tanstack/react-query' useQuery({ queryKey: ['some-query'], queryFn: async () => { // ensure you narrow before using error details if (Math.random() > 0.5) throw new Error('some error') return 42 }, // placeholderData replaces keepPreviousData placeholderData: (previous) => previous, }) // At call sites with global error = unknown, narrow explicitly before using properties. // Example: // if (error instanceof SomeError) { error.code ... } ``` Follow these rules to prevent accidental “error access” without narrowing, avoid UI bugs caused by changed status semantics, and ensure rollback and error handling occur in the right place.