Don't knit a patterned item when you are getting tired!
Ask me how I know - I think I have ripped out my new sweater at least five or six times. How could I not see that the pattern was shifting over a stitch half way up? I was beginning to wonder why I kept getting this wrong when it finally dawned on me. Each row begins with either knit 5 or purl 5 stitches, then it becomes either knit 4 or purl 4 for the rest of the row. If you are watching tv while doing this, it becomes very easy to knit 5 at some point across the row rather than 4 and then the whole pattern becomes skewed.
Grrrr ..... and I said I wasn't a perfectionist - guess I was wrong.