From my personal experience...

I recently had a flight from PVG (Shanghai) - BOM , the flight was via Delhi, no change of planes. We sit and wait for the passengers from Delhi to Mumbai. Now these passengers are 100% domestic. which means this is 100% domestic leg for them. (no customs or immigration after arrival at BOM). Now my question is that with these rules isn't it very easy for some one traveling from China to India to escape the customs ? I mean there might already be things like this been happening ? It is not (at all) difficult to make some one book DEL - BOM same flight as yours.

Just came to my mind and it seems like a serious flaw.

Experts please throw some light on this. I just pray nothing bad ever comes inside our country ever !