From Michael JG Meathook: Flashbacks
Stamford tells him of an old classmate of theirs who, so affected by the loud crash of the cymbals during a performance of La Traviata, started bellowing in the middle of The Royal Opera House. The poor man had believed himself back in Afghanistan, shrieking orders to those around him and sobbing uncontrollably.
Watson likes Stamford. He is still pathetically grateful to him for facilitating the meeting between him and his new flatmate, and was grateful for the invitation to dinner. He does not enjoy the way in which Stamford lingers over descriptions of the opera goers' reactions to the veteran's outburst with a smile on his lips, no doubt expecting Watson to mirror his amusement. Watson, ashamed, smiles blandly and swiftly changes the subject.
On the way home he cannot stop thinking about the story. Watson is lucky, he knows. There are many who have episodes like the man in the opera house, who find themselves suddenly plunged back into the battlefield as if they were really there. Watson has nightmares, but rarely anything quite so intense. Nothing that would give cause for anyone to notice the lingering effects of his time in the army, at least.
He steps into the Baker Street living room and Holmes looks him over with his usual interrogating gaze. Inwardly, Watson amends that there is perhaps one person who has noticed his nightly suffering.
"You are back earlier than I expected." Holmes's voice is carefully emotionless. "You won't mind my violin?"
Watson shakes his head no, then smiles to himself as he recognises the opening strain of a particular Mendelssohn favourite of his. That evening he drifts into dreamless sleep on the living room sofa.