1. Buying yarn without having a project in mind.
Unless you just want to stock up on yarn - I think it's best to buy yarn for a specific project. You may have the perfect pattern in mind when you go shopping and that's great, but you may find the perfect yarn without a project. I like to find a project for the yarn right away so I have a plan. Print out the pattern and keep the yarn and pattern together so you do not forget what you are going to knit with that beautiful yarn!
2. Forgetting to take notes when you change a pattern.
Yikes! If you have made changes to the pattern you are working on, you need to make notes of them or you most likely will not remember when it's time to repeat that part of the pattern, such as the other sock, the sleeve, the front piece, etc. If you keep track of what you do in Ravelry, you can make notes there so you always know what you did.
3. Not making a gauge swatch.
If you want your garments to fit, you need to take a sample. If you spend a little time doing this, your knitting or crochet project will be a success! You can also see how the yarn behaves; make sure it's perfect for your project. The most diligent pattern collectors block their patterns to get a good idea of what the finished project will look like. Just do it.
4. Starting new projects before the old ones are finished.
This point is controversial. Some people are "monogamous" knitters, working on one project at a time, and others ... are not. I am a prolific knitter and try to finish at least two projects a week - if I can do that, it's a great knitting week! If you are not a monogamous knitter, I recommend not having too many projects on your needles at the same time or you'll lose track of what you are knitting.
5. Working on a pattern you do not enjoy.
There is no rule that you have to finish a project you have started. If something does not work out the way you hoped, rip it out. You can keep using the yarn or even get rid of it if the yarn is the problem. Life is way too short to stress over things like this, and you will not believe the relief you feel when you make the decision to move on to another pattern!
6. Putting projects away without having the yarn and patterns with you.
It's so frustrating! The lonely half sweater wrapped in a bag with no pattern to be found. You had the best of intentions - "I'll just stash this here until I get around to it". And then you do not. When you finally find it, you have forgotten where you were in the pattern, what size you knitted, maybe even what the pattern is called! You might even have taken some notes on the pattern! My advice is to always keep the pattern with your project, even if you set it aside "just for a week or so".
7. Shrinking/felting of your projects due to improper washing.
This can be heartbreaking, and while I hope this does not become a habit for anyone, there are a few ways to avoid it.
Choose the right yarn for your project. If you are knitting for a child or for someone who may not know how to handle precious fibers, choose a washable yarn. There are many fabulous superwash wools, and cotton/bamboo yarns are perfect for warm weather projects. Sew in a care label! This is a great solution because people are used to looking for washing instructions on such labels, and you can save yourself the grief of a matted project. Remember, this is your first time knitting and you can always do the project again.
8. Playing "yarn chicken!"
When you are sure you have enough yarn to finish a project, but deep down you know you do not. So you go ahead and realize you need six more yards to tie off. We all want to try to use every bit of yarn, but it's important to make sure you have the right yardage for your project. You can always use those little scraps for "scraptastic" projects or for doll clothes.
9. Casting on without reading the entire pattern.
This is something I always encourage new knitters to do. You need to know what's coming next in a pattern, and you may need extra material, or you may see a new technique you need help with, so be prepared.
10. Avoiding finishing work.
Many knitters and crocheters hate finishing - so many seams! I consider them a challenge. I try to sew my projects as quickly as possible so I can wear my finished item! And honestly, I also choose patterns that require as little finishing as possible. For example, I love to knit sweaters in the round or make striped scarves where the colors continue around the edges instead of weaving in all the ends.