This is a very common tactic. I myself fell for it several years back. Never allow them to distract you. Also ask for a printed bill everytime. Now I've shifted to Shell. They issue the printed bill always and I can be assured that the fuel won't be adulterated. Its simply not worth the hassle just to save 20-25 bucks.
^^ haahaa same thing happened to me 2 days back. I went to IOCL petrol bunk near our township. Asked the guy to fill Rs.200 petrol. After pouring 1 ltr i.e 60 rupees he stopped and looked at my face. I told him "ok. 1 ltr is enough for me". Then he started asking me repeatedly Sir how much did u say!! I again told him 1 ltr is enough and paid 60 rupees. His facial expression was like OH ****!!!
If you ever top up your engine fluids oil or engine coolant at your regular fuel pump & there's a good quantity left in the can & the attendant offers to keep the half filled can in your trunk then please ensure that they have shut the cap or lid tight because the oil may spill all over the car leading to a hazardous situation & lot of wastage also. This has happened so consistently in so many cases & people tend to forget about it but can be risky.