This paper proposes a unified molecular mechanism for Short-Term Memory (STM) and the phenomenon of Déjà Vu. We challenge the traditional "Persistent Firing" model by arguing that STM is a transient structural state created by the Arc protein and erased by the enzyme Ube3A. We define Déjà Vu as a "Synaptic Cleanup Error" occurring when Ube3A fails to fully degrade Arc capsids, leaving a "Residual Arc" presence at the synapse. This residual protein lowers the neuronal firing threshold from 55mV to -53mV, causing a sub-millisecond "latency advantage". When a new signal fires "too fast" due to this lowered threshold, the brain misinterprets the speed as evidence of a pre-existing memory. This model explains why intense focus—which triggers high Arc production—often leads to both Déjà Vu and subsequent "Weak Memory" (encoding failure due to synaptic congestion).