The Chief Minister had reached the airport much earlier and had waited at the VIP enclosure at the state's hangar.
Closer to the time that PM Modi's flight was to land, he was moving towards the tarmac when the incident took place. Embarrassed, top Bihar police officers worked the phones to get the centre's Special Protection Group, or SPG, to let the Chief Minister pass.
The SPG relented, but it took a couple of minutes to resolve what could have turned out to be a public embarrassment.
The Special Protection Group, or SPG, which is mandated to guard the Prime Minister, forms the inner ring around the PM and strictly controls access to maintain a sterile zone in the immediate vicinity. That means it has the powers to stop anyone to keep the PM safe, irrespective of the location.
It is to avoid such situations, a Bihar police officer said, that coordination meetings are held by the SPG's advance security liaison teams that finalises PM Modi's itinerary from a security perspective.
That the faux pas still happened was a surprise, he said.
Asked, a senior SPG official attributed the incident to "some confusion" and stressed it was resolved quickly.
He said it is the norm, for example, that the Chief Minister would leave the airport before the Prime Minister does so that he is able to receive the Prime Minister when he reaches his next destination.
In this case the Patna Science College ground where PM Modi and Nitish Kumar shared the stage, the first time after Chief Minister swapped his allies and joined the BJP-led National Democratic Alliance.