You know when you're knitting away happily, only to look down and see, several rows below, a stitch that should have done differently. A purl bump interrupting your smooth expanse of stocking stitch, for example, like in the picture up above.
There's a few ways to deal with this. You could use an afterthought lifeline to rip back down, or you could tink back down all the intervening rows stitch by stitch. (You could also put the entire project in a plastic bag and stuff it at the back of the wardrobe, but let's assume that's not an option, shall we?)
If you only have to fix a single stitch, though, probably the best option is to drop down just at that place, fix it and then work back up. This involves dropping the stitch and making a ladder, and this sometimes makes knitters nervous, but here's how to do it with confidence.
The only tool you need is a crochet hook. (You can also do this fix without one, but even if you're not a crocheter, a hook makes a very useful addition to your knitting bag.)
Work across to the stitch that you need to drop...
...and just let it pop off the needle. Don't worry about it dropping - after all, that's what you want it to do. You might need to encourage it to head downwards if your yarn is at all sticky...
...and let it ladder down until it's reached the place you want to fix. (There's something rather liberating about this!)
Now pop your crochet hook though the loop of the stitch. We're ready to effect the repair.
Find the lowest rung of the ladder made by the dropping stitch - that's the strand of yarn that makes your corrected stitch. Pull it through the loop, from the back of the work to the front, without twisting it. That's the repair done, and now you only need to get back up to where you started.
To go back up, find the rest of the rungs of the ladder, and starting with the lowest, make new stitches back up, with each rung becoming a loop, which gets a rung through it, becoming a loop, all the way up.
When you've reached the level of the row you started on, all you have to do is pop the remade stitch back onto your needle, and continue as if nothing had happened.
See? There's nothing to indicate that this piece of knitting was ever any different, and if you don't tell, we won't either.
Completing a repair like this imparts a wonderful sense of warm smugness. You realise that, in Elizabeth Zimmermann's words, you are the boss of your knitting. And that feels very good indeed.