Signs You are Making a Knitting Mistake:
1) Too many stitches at the end of your row.
2) Not enough stitches at the end of a row.
3) The edges of the knitting are not uniformly straight, they seem to go wider to narrower.
4) Some stitches are sticking out more than others.
5) Some stitches look like they're too loose.
Check for Mistakes
Always count your stitches when you finish a row to make sure that you have the same number that your originally cast on. If you have too many or too few in that row you've either lost a stitch or dropped a stitch or accidentally added a stitch. That's the biggest sign you're making a knitting mistake -too many or too few stitches than you're supposed to have.
If you don't have the correct number of stitches, tink back on the row, which is to undo each individual stitch until you're at the beginning of a row. Watch carefully as you undo those stitches and see if there's any obvious difference between each stitch that can tell you what you may have done incorrectly.
Keep Your Yarn in the Correct Position for Knitting or for Purling
One really common knitting mistake that causes too many stitches is failing to keep your yarn in the right position when starting your next row. If you hold the yarn in the wrong position when starting your next row you can accidentally create an extra stitch. When you're knitting, you want to make sure that you keep your yarn at the back of the work. That means that when you insert your needle into the stitch to knit, you make sure that the working yarn is positioned behind the needle that you've just inserted into the stitch. When you're purling, you want to make sure that you keep your yarn at the front of the work, that is, when you insert your needle into the stitch to be purled, your yarn is in front of that needle. (see photos)
Keep an Easy Grip on Your Knitting Needles
Frequently check your grip on the knitting needles. It's easy for beginners to fear that the stitches will fall off the needles, but you needn't grasp the needles with the strength of an orangutan. Loosen your arms, shake out your shoulders, take a deep breath and go.
Make Sure You Pull Each New Stitch Completely Through the Old One
When you're unaccustomed to knitting you don't know what the action is supposed to look like, so it's difficult to tell when you're making a mistake. One mistake indeed is when you don't pull the new stitch all the way through the old stitch. You may lose your attention for a moment and think that you have the new stitch on your needle when you in fact have your old stitch which you have just simply slipped from one needles to the other.
Dropping a stitch is when you accidentally lose a stitch off of your needles and it unravels. If this happens you can easily pull the stitches through one by one using a crochet hook. You can see some examples here, or you can undo each stitch until you get to the place where you dropped the stitch. This is one way that you end up with the wrong number of stitches with your knitting.
Knitting two stitches together by mistake
When done in the right spot at the right time, knitting two stitches together is called decreasing, but when you do it when you're not supposed to, you end up with too few stitches. If you realize that you've done this, just tink back to the beginning of the row, count your stitches and try again. If you don't have the correct number of stitches, keep tinking back until you.
Not Switching the Yarn to the Front or back of the Work in Ribbing
Sometimes it's hard to remember that when you're knitting and purling that your yarn has to be in the right place. If you keep your yarn in the front like when you're purling and knit the stitch, you end up with an extra stitch. This is usually called a yarn over and you do it when you need to increase a stitch, but when you do it by accident you end up with too many stitches and ribbing that looks all wonky. The same thing happens when you purl with the yarn held in the back as if you're knitting. So when you're knitting one and purling one for a rib pattern, you have to switch the position of the yarn from that back to the front to purl, and then from the front to back to knit.
These are the biggest mistakes that you can make in your knitting and just having a little bit of knowledge and being proactive about counting your stitches will keep you from making too many knitting mistakes.
If you're having too much trouble teaching yourself to knit and need a little guidance, check out our learn to knit section and find ways to learn to knit online at home.