Failure Signatures - what the crash means and where the cause actually lives

Every runtime failure has a signature, and the signature names where to look - which is almost never the line in the top frame. This is the single-chat form of the diagnoser seat's failure catalogue: match the evidence to a signature, then isolate at the place the signature points, not the place it threw. It pairs with the superpowers:systematic-debugging method - that skill runs the disciplined hypothesis-and-test loop; this one tells you which hypothesis the signature warrants. Read the evidence first and quote the exact frame, then match.

The signatures - and where each isolates

• Null-reference / undefined access (NullReferenceException, Cannot read properties of undefined/null). The frame names the dereference, but the cause is usually one hop up: a dependency never assigned, or a value read across an async gap before its await resolved (an @Input touched in the constructor before ngOnInit, a field read before the task that sets it finished). Isolate by walking the assignment of the null symbol, not the line that dereferenced it.
• DI / composition-root failure (Unable to resolve service for type, a startup InvalidOperationException, Angular NG0201). The injection site is innocent - the registration is missing, mis-scoped, or a captive dependency (a singleton capturing a scoped service). Isolate at the composition root (the container setup / module providers), never the consuming class.
• Async deadlock / sync-over-async (a hang, not a crash). .Result / .Wait() / .GetAwaiter().GetResult() blocking a captured synchronization context (the classic WPF / WinForms UI-thread and ASP.NET-request deadlock), or a TaskCompletionSource never completed. Isolate to the sync-over-async boundary, not the innermost await.
• Race / shared mutable state (reproduces under load or a specific ordering, passes in isolation). A static or singleton mutated concurrently, an un-awaited fire-and-forget, subscriptions firing out of order. Isolate by naming the shared cell and its competing writers, not by re-running until green.
• Disposed / lifecycle (ObjectDisposedException, Cannot access a disposed context). A DbContext or HttpClient captured past its scope, a subscription or timer firing after teardown, a handler outliving its view. Isolate to the lifetime boundary that ended early, not the use site.
• Database contention / exhaustion (a query that deadlocks, times out, or violates a constraint under concurrency but is fine in isolation). A deadlock victim (SQL Server error 1205, Postgres 'deadlock detected') from two transactions taking locks in opposite order; connection-pool exhaustion (a timeout acquiring a pooled connection - usually a leaked or un-disposed connection or DbContext, or sync-over-async starving the pool); or a unique / foreign-key violation surfacing a lost race. Isolate to the competing transactions and their lock order, or the connection that was never returned - not the statement that happened to lose.
• Config / environment drift (a value read as null or wrong, no crash at the read). A missing user secret / env var / appsettings key, a connection string pointing at the wrong database, a WPF binding silently no-op'ing (the System.Windows.Data error in the Output window). Isolate by proving the value the code actually received, not the value it expected.
• Boundary / off-by-one (IndexOutOfRangeException, ArgumentOutOfRangeException, an empty-sequence .First() / .Single()). A fencepost in a slice or loop, an empty collection assumed non-empty. Isolate to the boundary input that triggers it, and keep it as the regression case the fix must cover.
• HTTP error with a clean response (a returned status, no stack trace). Triage by the status class - it names the failing layer before you open a trace: 401 authentication, 403 authorization / policy, 400 / 422 validation, 404 routing / binding, 405 verb, an antiforgery or CORS-preflight rejection. Isolate at the layer the status names, not the handler body.

How to use it

Match the evidence to one signature, form the fewest hypotheses it warrants, and confirm each against located code before you touch anything - root cause before symptom, never a plausible guess. State the match in three lines - the quoted evidence, the signature, the isolation point (and the hypothesis it warrants) - then run the loop: