There is a famous (but for the life of me, I can't remember who it's by!) fictional short-story about an 'open day'(!)/press-event/family-picnic-thing at a missile launch silo where someone is being shown the control room: "What we'd do, if we wanted to launch is to do
this... and
that... and this
other thing, but don't worry, because it needs the silo doors to be open (etc)", intermingled with a photographer being helped to get a good photo on the surface ("it looks a bit boring, can we open the silo doors...? Ah, thanks...", etc, narritively intermingled) and all alongside a firework display. 'And then the biggest firework of all went up!'.
Not that this is how it happened this time. I'd imagine they were testing some of the launch circuits (e.g. green light showing for the stretch from control-room to rocket-platform) and didn't know that some of the further in-series circuits (from the platform to the rocket itself) weren't disabled/disconnected. Or they accidently tapped the real-codes instead of the drill-codes into the launch controller, for similar not-non-effect.
Obviously, the shooting match didn't spark off from this event, but it didn't take the missile actually delivering its payload (or not) to provoke a response, just its detection in flight (or, if their monitoring was good enough, the unscheduled/not prewarned-about launch itself) that could have triggered the thought that there was a First Strike in progress, to which a retaliation (initiated before the first missile actually hit any facilities/C&C locations that it might be destined to arrive at) is a possible knee-jerk response.
As with the legendary soviet controllers/captains who are credited with refusing to believe that an erroneous launch-detection/war-escalation was actually The Real Thing™, and probably also some similarly fortunately hesitant individuals in equivalent positions in other countries' system of launch-control, it may have been as little as one person's incredulity that stopped things escalating, at least until the Indian forces got on the hotline to the 'enemy' to quickly convince them it's not what it might look like, giving the opportunity for people to act more sensibly in the light of the error.
(Obviously that needs that everyone is not on a knife-edge, already, or the
Fail Safe system might fail-dangerous instead.)
...anyhoo, as it happened a few days ago, and it resolved nicely for all concerned (and also for those it did not), we can say that it wasn't a big enough error to complain about, but still worrying and the promised enquiry might well be worth reading, if it ever gets published without too much redaction to it.