We have a spooky sweater in our house that is haunted; it is actually 1 sweater plus the 2 sweaters that I unravelled. It was a sweater for Charles. The first one was a top down seamless raglan with stranded colour work around the yoke. I got 75% done, and realized the whole thing was boring and the yarn was kind of wrong for a guy's sweater. Frogged it. Next one, added a strand of sock yarn to the yarn for more oomph, redesigned it, now a seamless bottom up raglan hoodie with a kangaroo pocket, with seed stitch panels and cables. Got ALL THE WAY TO THE NECK, and realized (because it became really obvious when I got to the neck) that the back panel of seed stitch, which was supposed to be centered, was OFF CENTER. Not enough to make it look intentional.
The third time, I did successfully finish the sweater, but the magic was really gone. It looks fine, but honestly, the yarn isn't right for a large men's hoodie, it stretches.
I don't know if other knitters have this problem, but I make a lot of dumb mistakes. Dumb because they're PREVENTABLE. I did it again recently; again on a seamless garment, so the rows are really long. Got to the underarm shaping, and realized that I had missed the side shaping, but only on one side. How do these things happen?
So here's a mantra I'm making for myself on how to avoid knitting mistakes, maybe this will help others too:
Count your stitches
Count them after you cast on. Count them after you increase or decrease. Sneak up on them and count them randomly; you are the tax auditor and those stitches are up to something. If you're working a seamless garment, count all the sections (sleeves, front, back, button bands, lace panels etc).
Why do it: Checking your stitch count is the best preventative medicine there is. You will catch simple errors, like casting on one the wrong number of stitches, but it will also catch skipped shaping, dropped stitches, a missed yarn over.
Measure stuff! Measure it accurately! Measure the clothes you wear. Measure your body. Read the pattern schematic. Measure your swatch. Measure your knitting as you're working.
Why do it: Measuring the clothes you actually wear against your own measurements and the pattern schematic is invaluable. If your favourite sweatshirt is 26 inches long, but the pattern schematic says the sweater you're about to make will be 19 1/2 inches long, take the time to adjust the pattern.
Measuring your gauge assures that the thing you knit is the size stated in the pattern. Tiny differences in gauge will make inches of difference.
Measuring as you go keeps you on track and lets you adjust length or width if you need to before it's too late.
Take a few minutes, and read through the pattern. Check for things like "at the same time". This applies especially to seamless patterns. Check for how things are going to line up. Check for tools that you're going to need.
Why do it: Seamless patterns are popular because they are easier to finish, you can try them on before they're done, and don't have bulky seams. But seamless patterns often do a lot of things at the same time. On any given row, you might, for example, be working a button band, a button hole, raglan shaping, and a cable.
Keep a pencil and notebook handy and make notes. I count my rows as I go, even if I don't need to. Just a little notch. Or if I'm watching a movie and I know that I will have to work a buttonhole in 3 rows, I make a note "button hole in 3 rows!"
Why do it: It helps to avoid mistakes in shaping. A lot of patterns say "Inc every xth row y times." So it helps to know exactly what row you're on. And it helps to match things up; if you work 1 1/2 inches of ribbing for the cuff, and it's 12 rows, then you'll know to work 12 rows on the sleeve cuffs too.
Patterns worked in the round usually say "join for working in the round, being careful not to twist stitches." And if you have ever twisted your cast on, you will know why this reminder is always there. If you twist the cast on you end up with a Moebius strip. Mathematically fascinating, but not useful for a hat or sweater. Impossible to repair without surgerical intervention. And in a long cast on, very easy to miss, even after several inches of knitting. So check it when joining, then check it a few rows in. (added December 5, 2012, after making this mistake)