Sync-over-async thread starvation
A C#/.NET async web service (ASP.NET Core) that normally handles 30k RPS on a small thread pool starts collapsing under a moderate load increase: throughput craters, latency climbs to seconds, and the runtime keeps slowly adding threads (you see the thread count creeping up over minutes and brief recoveries each time a thread is injected). CPU is only ~40%. A recent change added a call into an older library method that is synchronous and does network I/O, wrapped as `SomeLegacyClient.GetData()` and called with `.Result` / `.GetAwaiter().GetResult()` from inside an async handler. How do you triage and mitigate?
Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.