Endpoint descriptors, firmware queues and controller configuration can disagree after endpoint-zero enumeration. Trace the actual endpoint/direction and its buffer readiness.

Common trap: Giving a generic fix without identifying the boundary and unmet obligation in this example.

Correlate attach/speed-negotiation state, reference clocks, reset, controller status, firmware progress and a bus trace. No observed bus activity makes host scheduling, reset/clock/power, attach signalling and the observation point candidates. SETUP without a response narrows the path to endpoint-zero/controller/firmware handling; STALL during a descriptor request requires decoding the request and endpoint state rather than assuming a descriptor mismatch.

Common trap: Rewriting the host driver first.