He topped up. The toll still said low balance.

transcript

His son topped up the FASTag. Twelve minutes later, the toll said no. The list it checked was two minutes too old.

11:42. Rajesh's wallet runs low. His tag goes on the low-balance list. 11:50. The next plaza gets that list. 11:52. His son tops up 5,000 rupees. 12:04. The lane checks its copy. Low balance.

The online check could have saved him. But the link was slow. So he pays by UPI. 1.25 times the fee.

Here's the catch. That list is a cache. Changes reach each plaza as a diff, every 10 minutes. So every lane's copy is stale for a while. Always.

So NPCI priced it. After listing a tag, the issuer stays liable for 20 minutes. Listed over an hour before the read, and still listed after? The switch declines it. Code 176. The plaza's side takes the loss.

You can't keep a thousand plazas perfectly fresh. So write down who pays for every stale minute.

Full breakdown in the lecture.