1) Are you forgetting to drop stitches from the left (secondary) needle?
This is probably one of the most common mistakes. Knitting is an exercise of concentration and dexterity, and usually it takes a good deal of practice before someone is able to knit on “auto-pilot”. As such, when someone is learning how to knit it’s very easy to forget dropping a stitch from the secondary needle, as soon as it has been knit – which will eventually lead to the same stitch being knitted twice, which in turn will lead to the creation of extra stitches.
How to avoid doing this: remember, knitting involves four base movements: insert down the hole, around the backside, trough the loop and let go.
2) Does the first stitch of a row actually look like it’s two stitches? As you probably know, you always have to move the working yarn around the back when you begin stitching a new row. If you accidentally take the yarn up and over the needle, you’ll be setting yourself up for trouble! Doing so will pull the last stitch in a row across the needle, which will make it look like it’s actually two stitches. Needless to say, this is a recipe for disaster, since you’ll have gained an extra stitch – which isn’t something you normally want to do while knitting.
How avoid this mistake: Every time you finish a row, make sure you double check the yard was pulled across from under the needle, and closely notice whether the first stitch looks right.
3) Is your yarn splitting in double threads, somewhere? Unfortunately, this is something that tends to happen even when you only use expensive top-quality yarns. Getting the plies of the yearn untwist while you need is excessively simple, so there’s a chance it will end up by accident if you’re not watching it closely. When such a thing happens, it will make a larger stitch look like two smaller “half-stitches”. You should make sure to keep inspecting your loose ends of yarn as you go along, to keep your threads from splitting.
How to fix this problem: Try using different kinds of yearn; tight and evenly spun yarns work best to avoid this sort of trouble.
4) Did you happen to make an accidental yarn-over?
When you need to make holes as well as extra stitches, it’s very common to make a yarn-over by mistake. This is what happens when the yarn you’re working with is wrapped on the needle, making the next row to get knitted as though it was a stitch. This type of mistake is sometime referred to as “knitting purlwise”, and in simple terms it involves knitting the wrong side of a stitch.
To avoid doing this: you should make both needles go through in the same direction, while knitting a stitch.