A similar question was recently asked by my newest knitting buddy, Mrs. U. (She asks a lot of great questions!)
Have you ever started a project with the correct number of stitches cast on and several rows later had a widening garment, unintentionally? Have you ever tried to figure out how it happened and felt lost? Boy, I have.
Let's talk holes. If you look back and find holes where there aren't supposed to be any you have yarned over. A yarn over (yo) is a stitch used to increase. If you yarn over the first or last stitch on a row you will increase without a hole. However, a yarn over in the middle of a row will leave you with a neat little hole. This "mid row yarn over" is often used to create lace-style holes for decoration or as a small part of a series of stitches for a specific design.
The yarn over stitch is very simple. You simply loop your working yarn around your working needle and continue knitting stitches as before. This is what caused your unintentional hole. You probably thought you had your needle properly inserted into the stitch you were trying to work and, for whatever reason, it wasn't inserted correctly. Then you looped the working yarn around the working needle and pulled it through as usual you created a yarn over instead of a completed knit stitch. You may have even pushed the unworked stitch off as worked creating a dropped stitch, which made an even larger hole.
To correct this error go back and pick up the correct stitches. Yes, this means you have to unravel to the error and work this section all over again. This is a great reason to always check your progress every row or every few rows. (Sometimes you can't see the error until a few rows later.)
If you find no holes you have neglected to drop your worked stitches. Once you work a stitch you normally drop it off the end of the needle and continue on to your next stitch. You may have thought that you dropped you stitch and somehow didn't. Occasionally, especially with fluffier yarn, the yarn will catch between the worked and new stitch and the worked stitch will carryover to the working needle. Sometimes you're working so fast you just don't notice this!
If this is your problem you can easily spot these undropped stitches before reworking them on the next go round. Watch for crossed stitches that making an "x" on your needle. When you come to an "x" work the first stitch as normal and drop the second stitch off the end of the needle without working into it. This drops the original offender stitch back into its proper place and allows you to continue on as if you never had a problem.
Are you a knitter that multi-tasks while knitting? Generally, you can find these errors by feel. When attempting to knit into a crossed stitch you'll notice that the first stitch will feel tighter than the others around it. When you feel this tension make it a habit to look for a crossed stitch.
The only way to correct the increase errors several rows down is to back down to the problem spot and rework all of your stitches. Should you only be a row or two ahead you can try a knit two together (k2tog) decrease to get you back to the correct number of stitches. To knit two together simply push the working needle through two stitches rather than one and push both worked stitches off the end. (If you find yourself more than 4 stitches ahead, you may want to back down to rework all of the stitches anyway, as decreasing too much will leave you with a wavy end product.)
Hope these tips help you!