My first job was at a grocery store, as a cashier.  The chain I worked for was great in terms of pay/benefits/flexibility/etc., but there were definitely some trends in customers that were universally deplored or appreciated.

Little things—like checking to see if the cashier's light is on before you start stacking your groceries, or having an actual conversation with the cashier, or being nice when something goes wrong—don't require much from you, but really make a difference to us.

Similarly, if you yell at us for something that's beyond our control, or are rude for no real reason, it makes our 8-hour shift that much harder to get through.

In my case, the worst thing a customer could do was do something that would slow down the scanning process, like running back for something or changing their minds about buying an item.  Once the first item is scanned, the computer starts calculating your IPMs (Items Per Minute).  Some chains don't care much about IPMs, but some do, and at my chain we could get major bonuses (or demerits) depending on the state of them.  If you can avoid screwing those up, you may very well go away having brightened some poor cashier's day.
user
If you care about your IPMs (Items Per Minute), don't be a dick. If you don't care about IPMs,
assistant
My first job was at a grocery store, as a cashier.  The chain I worked for was great in terms of pay/benefits/flexibility/etc., but there were definitely some trends in customers that were universally deplored or appreciated.

Little things—like checking to see if the cashier's light is on before you start stacking your groceries, or having an actual conversation with the cashier, or being nice when something goes wrong—don't require much from you, but really make a difference to us.

Similarly, if you yell at us for something that's beyond our control, or are rude for no real reason, it makes our 8-hour shift that much harder to get through.

In my case, the worst thing a customer could do was do something that would slow down the scanning process, like running back for something or changing their minds about buying an item.  Once the first item is scanned, the computer starts calculating your IPMs (Items Per Minute).  Some chains don't care much about IPMs, but some do, and at my chain we could get major bonuses (or demerits) depending on the state of them.  If you can avoid screwing those up, you may very well go away having brightened some poor cashier's day.
user
Don't be a jerk, and leave your customers happy.
assistant