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Having tilled the soil for about 50 years now, I have made plenty of gardening mistakes myself, and seen ones that other people have made.  So I’ll try to distil this article into some of the key ones.
 
Mistake 1: Choosing the wrong vegetable site
Don’t just go for the patch of land that is at the back of the garden or up against a hedge or fence. Think about the following first:

 

Is the site of your potential plot going to get the maximum of sun and the minimum of wind (especially cold winds from the north)?   Warmth and sunshine are major factors in the promotion of healthy fruit and vegetables.  They're just as important as good quality composted soil.
 

Is your plot going to be a ‘safe’ distance away from those ‘slug and snail’ jungles, such as thick bushy or weedy growth, rocks or other piles, or even a fence (what’s on the other side of the fence?) 

 
How close do you want your vegetable patch to be to the kitchen?  Do you really want to traipse all the way to the garden plot for just a few leaves of lettuce?  Maybe some of your garden can be closer to your house, with other bits farther away?  The plot doesn’t all have to be in the same place.  Grow small plants like lettuce, parsley or basil in pots near the kitchen door.

Are your plants going to be reasonably well protected from pets so that your vegetables will be trample-free?  What about children?  Are small feet or badly kicked footballs likely to land on your seedlings?

  

Mistake 2:  Forgetting about your back

No matter how fit you are, gardening can be very hazardous to the health of your back, especially when digging or weeding.  As the Garden as Gymnasium article says, gardening is a great way to get exercise and become quite fit.  But, just as you can get shin splints by jogging over the wrong  type of ground,  you can get serious back ache by doing some gardening activities the hard way. 

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If your soil is not too crumbly, then use a pitchfork instead of a spade to turn it over.  So much less of a strain on your back.  This is especially true of soil that has yet to be turned over. A pitchfork penetrates the soil so much easier than a spade.
 

Never dig soil when it is wet.  Not only will it harm the soil’s composition, but wet soil is much heavier than dry soil.

Use a kneeler when weeding or thinning out your veggies. Hardly any strain at all on your back. And if your kneeler has vertical handles on each side, all the easier for you to get upright again, and easier on your knees as well.

 

Mistake 3: Buying too many seeds of one plant, and not enough of the other

It is very easy to buy too many lettuce or radish seeds, and you end up with far too many left over.  Note how many seeds tend to be in the package.  The average family only needs about 5 – 10 healthy lettuce plants in any given week, not 50.  And why cut the whole lettuce plant if you only need a few leaves?  Just take the leaves you want and leave the rest of the plant in peace. Same applies for spinach or cabbage.

 

Similarly, you don’t need more than 25-30 radish plants at a time.  You can do succession sowing (sowing seeds every 3 weeks) to spread out the crop’s length, but you can only do this twice or three times in a season.  One package of radish seed is enough, and should last you for two years.

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You only need about 6 – 8 courgette (zucchini) plants at most in a given season. It’s a good idea to sow a few extra plants, as they can be difficult to germinate.  Same rules apply to cucumbers.

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However, you can never have too many tomatoes, cooking onions or potatoes. The latter two will last a long time in the soil, as will carrots. And people tend to consume these in abundance. You also need at least 75-100 plants of sweet corn to make it worthwhile.  If there are 4 of you at a meal, then you probably need about 8 cobs of corn for that dinner alone.  And, more often than not, one corn plant will yield only one decent-sized cob of corn.

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Also, remember,  F1 seed varieties are costly, and you only get relatively few seeds in each package.  You may find that they are not value for money.

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Keeping a garden diary will help you to refine your future seed orders.

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Mistake 4: Sowing every type of seed directly outdoors

Many seeds will germinate more readily if started in a seed tray on a southern window sill, a greenhouse, or even an airing cupboard (especially if near a heat source like a hot water tank. And when you plant out the seedlings, they have already had a good start against pests, and you can plant them exactly where you want them.  If you want nice and tidy rows,  it is so much easier to do so with already germinated seedlings than by direct sowing.

 

Seeds that can tolerate cold spring weather (like peas or broad beans) can be sown directly outside, but cut your odds and maybe sow some in pots in the greenhouse.  Same for corn.

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Delicate seeds like lettuces and parsley tend to germinate better in seed trays on a window sill or in a greenhouse.   Big seeds like beans or squash plants can become prey to mice, even if in a greenhouse.  The mice just dig them up and eat the seeds.  Squirrels are known to do this as well. To stop these predators,  just put a sheet of glass or plastic over the seed tray until the seeds germinate.

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Seeds like tomatoes, cucumbers and courgettes really want a consistently warm soil to germinate.  If possible start them off in seed trays in an airing cupboard. But check them for germination after 3 days, and don’t leave them in the cupboard after the first signs of the seedlings breaking the soil. Once they break the soil, move them to a south-facing window or greenhouse. (If instead you leave these germinated seeds in the cupboard, they will frantically stretch for light, and you will end up with pale elongated and weak-stemmed seedlings in just 1 day.)

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Normally, root vegetables like carrots or beetroot do not transplant well, and should be sown directly outdoors.  But you can do it if you plant out the seedlings before they are too large (ie about half an inch to an inch in height), and you are careful to put the roots into deep enough holes. (I just stick my finger in the soil for each seedling, drop the roots into the hole, and then gently firm up the soil against the roots.)

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Mistake 5: Growing your plants too far apart.

If you have good nutritious soil, and your plot has good exposure to the sun, then you don’t need to grow your vegetables as far apart as the seed packages say.  The plants need light and air, and adequate root space,  but if you grow shallow rooted vegetables (like lettuce) close to deeper rooted vegetables like carrots, then they can be quite close together.  Just make sure you can get to them for any weeding or harvesting.

  

A small square or circle of vegetables might be better than several rows of them.   Why do this? Two main reasons:

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Densely-cropped vegetables allow little breathing space for weeds. Destructive predators (e.g. like carrot fly) will find it harder to locate the vegetable they want (e.g. the carrot) if it is surrounded by other kinds of vegetables.

 

But check out the companion-planting guides.  Some vegetables do very well together (like tomatoes and basil plants) whereas others do not (e.g. tomatoes with beans or potatoes, or spinach with peppers.)

 

Some plants, however, do need a lot of space.  Courgettes are particularly hungry for space, as well as nutrients.

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Mistake 6: Letting taller plants block out the sun for smaller plants

Common sense, I suppose, but in your planting design, make sure that tall plants like runner beans or sweet corn are not going to shade smaller plants which need lots of sun, like beetroot or onions.

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Mistake 7: Over-watering

The smaller the seedling the more important it is that the soil does not dry out.  But once the plant is reasonably well established in the garden, cut down on the watering and let the roots find the moisture.  In very hot weather, plants will need more water.  Then it is important to give the soil a good soaking.  Just sprinkling water on the plants does more harm than good.

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That’s enough rules for now!   But hopefully they are enough to help improve your chances of obtaining a successful garden season.