Everyone makes mistakes at one time or another, some of us make many, many mistakes and most of us would love to have a "do over". In golf, they call this a "mulligan" where the bad shot doesn't count and can be taken again. There are so many do overs that I would like to have in all these years that the count would be too high to calculate.
Fortunately, in knitting when you make a mistake you do get a "do over". That is what we call "frogging" which is taking out the stitches done incorrectly and knitting them over again. So if you have miscalculated a pattern, dropped a stitch or counted your stitches wrong you can take out the stitches up to the error and then correct it. Of course, unless you find your error very early, then you have a lot of frogging to do. The link below is from Knitty and explains frogging in detail.
KNITTY ON FROGGING
Now how to figure out how to "frog" the rest of life's mistakes.