If you are in the market for a new greenhouse kit, make sure you do your homework.
You want a greenhouse that works for your needs and you want one that just plain works.
The first thing to do is get clear with yourself on a few things.

1. What do you want a greenhouse for? Expanding space for your house plants, extending your gardening season, and year round food production are the main reasons people build a greenhouse.
   
2. How big a greenhouse do you want? How much space do you have to put a greenhouse in? Knowing what you want a greenhouse for helps to answer this question.
   If you are looking for year round food production, you will need a bigger greenhouse than if you are expanding space for house plants.
   
3. Are you serious about owning and maintaining a greenhouse? Often they turn into sheds or worse.
   If you aren't sure, it might be OK to buy a cheap one, otherwise get one with glazing that will last at least ten years and be sure it has a decent warranty.
   
4. How handy are you? If you are going to put it together yourself, know what you are getting into.
   Even if you are paying someone, you don't want to spend hundreds of dollars in labor on a difficult to erect greenhouse kit.
   Some are easier than others.
   Some fairly high priced brands are harder than they advertise.
   

Now that you have taken a good look at where you are at, we can look at the most common mistakes, some of which I have already hinted at.
Mistake #1: Buying the wrong size greenhouse kit.
Most people I know started out with greenhouses that were too small.
The opposite is also common.
If you think you want a big greenhouse, but you aren't sure, get one that is extendable, and plan on how you will extend it when you get to that point.
Being clear on what you want the greenhouse for should help you figure out how big it should be.
Talk to other greenhouse gardeners and find out what works for them.
Mistake #2: Buying Greenhouse with the wrong accessories.
The biggest one here is shelving.
Some greenhouses come with and offer shelving as options.
If you are growing houseplants or starting lots of seeds in your greenhouse, you want lots of shelves.
If you are mostly growing in the ground, you don't.
If you need them it's usually cheaper to get them standard.
If you don't need them don't pay for them.
The other thing is automatically opening vents.
Make sure you get them or you risk baking your plants.
Mistake #3: Buying a greenhouse with cheap glazing.
If you plan to keep your greenhouse for more than two years, don't use plastic film to glaze your greenhouse.
It doesn't last any longer than that.
Aside from that, make sure your greenhouse glazing has UV protection.
Except for glass, all glazing materials break down in sunlight.
Also if you are using your greenhouse in the coldest months, you want glazing that hold the heat in.
Mistake #4: Buying a greenhouse kit that is a pain to put together.
Really do your research on this one.
The ones that are harder to put together than the manufacturers say, usually leak a lot of air as well.
You want a nice air tight greenhouse.
Being careful about your research will help you enjoy your greenhouse for many years to come.