I realized the other day when I was at the pump, lifting the nozzle and selecting 92 octane, that a good amount of the fuel I'm about to pump could well be a mix I didn't bargain for. I thought about all the fuel sitting in the hose and the meter, and wondered if the previous buyer had gone for 87, as is most likely. So all in all I could be buying a couple litres of 87 with my 92 on my 5-7L top up.
So, and my apologies for sharing this random thought , I now stick to fuels with their own specific nozzle, ala Chevron 94 (and 92 at many stations, like Shell). That way at least you know what you're getting. Perhaps somebody with a better knowledge of gas pumps than I can comment on how much 'slack' is in the lines and you recieve. In a car this wouldn't be a real consideration given the volume, but in 5 litres it could well be.