Become a fan of h2g2
Supermarkets are very annoying. Avoid them if at all possible. Get someone else to go there for you, and make sure they have an important appointment to go to straight afterwards so they don't have time to hang around telling you what a horrible time they had while in the supermarket on your behalf.
Never say, 'I'm just dashing up to the supermarket - does anybody want anything?' because your 'quick dash' will turn into a marathon, and you'll find yourself approaching the checkout with an overflowing trolley.
What is a Supermarket?
Supermarkets, for those that don't know, are primarily places to go to buy food. They should not be confused with fast food outlets1, nor with small corner stores, which are - naturally - much smaller than most supermarkets2.
The worst kind of supermarkets are the ones that sell things other than processed food. Some product diversity is essential, of course. When people say they are going 'grocery shopping' they usually expect you to know that they mean food plus general household things such as toilet paper, washing up detergent and carpet deodorising powder; but when going to the supermarket means you no longer need to visit the hardware store, the chemists or the greengrocers, you're not only missing out on some marvellous cultural experiences, you're setting yourself up for a whole string of stress-related diseases.
Most major supermarket chains these days have a bakery section and a fresh produce section. This is insane because it takes away customers from the small bakeries and greengrocers that supplied people with bread/buns/cakes etc. long before anyone ever thought of supermarkets. What happens then is that those people go out of business, shut up shop and move away, and into their vacated premises will move something horrible - like a coffee shop specialising in teeny-weeny cups of coffee3. The people who like to drink these teeny-weeny cups of coffee don't live in that suburb, so they have to drive to the coffee shop and park their cars anywhere they'll fit, thus taking away parking spaces from the people who really need it, ie people who are doing the weekly shopping and don't want to have to take their 17 bags of groceries home on the bus.
Many people take a list with them when they go to the supermarket. This can't be just any old list; it must be a list of stuff they want to buy from the supermarket. Inevitably they will forget to put on the list the one thing they really need, or they will put it on the list but still forget to collect it on their way around the aisles. In both cases they tend to remember the forgotten item when they are in the checkout queue, but usually they will just swear a bit because going back to get it means they will have to wait for another 30 minutes in another queue4.
On the rare occasion that the person does decide to go back for the forgotten item, either there will be a trolley snarl that prevents them from getting into the necessary aisle and they will give up and go back to queue at the checkout, or when they get to the section where the product is normally found, there won't be any left.
The people who don't take lists are either very lucky (because they have a nearby parking space and lots of money, so it doesn't matter how many impulse purchases they make) or very annoyed with themselves for forgetting to bring one5.
Some people are on a budget. This means that they have a limited amount of money to spend at the supermarket. People with children often pretend their budget is much lower than it really is in order to accommodate the things they feel they have to buy for their children just to get some peace-and-quiet on the way home6.
It's easy to spot the people who really do have a budget and are serious about keeping within its limits - they will keep a running tab on the value of the items they have thus far selected.
At the Checkout
Once you've made your selections, navigated the trolley snarls and waited patiently in the queue (assuming you didn't go back for something, in which case you've waited twice), what inevitably happens is that one or more items won't scan properly. This means that the price won't come up on the register, and the checkout person has to apologetically ask you if you happened to notice the price of the item. Since anything you really want or, possibly, can't afford never has a price tag, and anything you're not that fussed about was just tossed into the trolley without even looking at the price, the answer is almost always 'no'. The checkout person then has to summon another of the supermarket's employees to go find out what the price was.
It's better to tell the checkout person you won't worry about that particular item - ie leave it behind - than to bear the angry looks and unfriendly comments from all the people waiting behind you who don't want to be held up while your price check is done. Trust us on this one.
Once you've had all your purchases scanned and have paid the checkout person, there is still one more problem - how to get it all home. If you have a car (or other means of personal transport) you're okay (assuming you can remember where it's parked). If you have to get home on the bus or train, or worse, on foot, you have problems. Things in cartons and boxes will bruise your legs, things in tins will mysteriously punch holes in the bottom of the bag and fall out while you are crossing the road (and either get run over by a bus or roll down a drain), things in non-rigid packaging (such as biscuits) will get crushed or squashed by things in tins...
See why it's always better to get someone else to go for you?