Sometimes when you examine your work, you will find an error or a dropped stitch. These things can be corrected with patience and a crochet hook.
To pick up a dropped knit stitch, insert the hook through the front of the loop and pull the laddered yarn from back to front. Continue up the ladder until you can put the final loop back on the needle. To pick up a purl stitch, insert the hook through the back of the loop and pull the laddered yarn from front to back. Continue up the ladder until you can put the final loop back on the needle. An alternate way to correct a purl is to turn your work over and treat it as a knit, correcting as described above.
To correct an error, trace the error up the column to the needle and drop the corresponding loop. Gently pull out the column of stitches until you have pulled out the error. Correct it as a knit or purl as described in the paragraph above. Although much easier if the error is only a few rows down, with patience and care you can correct errors pretty deep in your work.