You are told: a control pulse crosses into a slower clock domain through what the designers describe as a synchroniser. Every simulation passed. In bring-up, one product variant hangs intermittently during initialisation, only under voltage droop combined with high temperature. The hang rate is low: most boards boot, some do not, and failing boards usually pass on retry.
Three details carry the diagnosis. First, corner-dependence: droop and heat slow transistors, so a failure that appears only there implicates timing margins, not function. Second, intermittence with retry recovery: each boot samples a different phase alignment, so some boots are lucky. Third, the crossing in the path: a control pulse into a slower domain is the textbook lost-event shape, and "a synchroniser" without a scheme name is an unverified claim. The most likely mechanism is not metastability at all but a pulse the slow clock misses when margins shrink, possibly combined with a synchroniser that was never width-checked.
Assuming random noise without a crossing audit: noise does not hang one variant at one corner repeatably.
Relying on two flops for a one-cycle pulse without width analysis: two flops settle the value but cannot recover an event the first flop never saw.
Skipping formal and liveness checks after the RTL patch: the patch changes the protocol, so the proof must be re-run, not assumed.
Does the hang correlate with voltage and temperature corners across boards? Noise does not track corners repeatably; a margin-dependent crossing does.