You may have stopped working on the front in the middle of a row and put it away. When you picked it up again you may have started back over the stitches just worked instead of finishing out the row. The simplest way to avoid this is to finish a row before stopping and putting the piece away. It is like putting in an inadvertaant short row.
You can also avoid dropped sts when you frog by pulling out the yarn stitch-by-stitch on the last row (tinking). That way you can put each stitch onto the needle as you pull the yarn out. This video shows it for sts still on the right needle but you can also do it with the right needle removed along the entire row.
Or you can insert a needle into the destination row.