>> "Synchronize all external wire inputs with two clocks before using them."
This is why you need to sync external (async) inputs:
"Metastability in electronics is the ability of a digital electronics system to persist for an unbounded time in an unstable equilibrium or metastable state.
In metastable states, the circuit may be unable to settle into a stable '0' or '1' logic level within the time required for proper circuit operation.
As a result, the circuit can act in unpredictable ways, and may lead to a system failure..."
"Metastability in electronics is the ability of a digital electronics system to persist for an unbounded time in an unstable equilibrium or metastable state.
In metastable states, the circuit may be unable to settle into a stable '0' or '1' logic level within the time required for proper circuit operation.
As a result, the circuit can act in unpredictable ways, and may lead to a system failure..."