I remember some years ago I got pulled over just after lunchtime on my motorcycle with a friend on the back, I was bagged and indicated 0.07% Blood alcohol Concentration when the legal limit for me at the time was 0.02% I was very nervous as I knew the law and also knew that I had not touched alcohol in days.
I explained this to the police officer and he did seem a little puzzled as I did not indicate any degree of obvious impairment.
He allowed me to go and wash my mouth out with water at a nearby tap and less than 1 minute after initial test I blew in the bag again indicating 0.0%
In short, mistakes can be made, but even the most "police state" tests do allow for such anomalies (as the Indian policy states)