You are told: a control block takes an interrupt from a sensor clock and processes it on a slower clock. In the field, roughly once in weeks of operation, the control FSM enters a state the designers call impossible, and the system freezes. Power-cycling recovers it. The bench has run the same test for a month with no failure, and simulation is clean. Nothing else is known.
Three facts matter and everything else is noise. First, the state is illegal, which points at the FSM sampling something neither cleanly 0 nor cleanly 1, or at two related inputs disagreeing. Second, the failure is rare and unreproducible, which points away from deterministic logic bugs and toward timing-dependent capture. Third, a clock boundary sits exactly where the suspicious input enters, which makes the crossing the prime suspect. Repeat these three back before theorising: it shows the interviewer you diagnose from evidence.
Do not assume the synchroniser is present: "there is a two-flop synchroniser" is the most common invented fact in this scenario.
Do not assume noise or a faulty board: the illegal state implicates the design until the crossing audit clears it.
Do not assume simulation coverage means anything for this failure: rare capture-timing failures are exactly what simulation cannot prove.
Ordinary logic bugs reproduce deterministically on the bench; a rare illegal state at a clock boundary matches a capture-timing failure that needs an unlucky phase alignment, which happens rarely and never on demand.