The rules governing personal conduct on the top deck of a Glasgow bus are known to international peacekeeping forces everywhere: keep your head down; do not make eye contact and on no account attempt to make polite conversation with strangers. Refrain too from making jerky or unpredictable movements. It was probably the last of these that I transgressed last Thursday evening as I fished out my electronic cigarette and started puffing. Yet only for a few seconds did subsequent events threaten to get out of hand.