You have bought yarn, pattern, maybe buttons and a new needle. It's tingling in the fingers to get started. 23,000 stitches later, you can see that there is something that may not be quite as it should be - but what the heck - you have followed the pattern so it should be fine... but no, after 55,000 stitches you realize it didn't. Your knitting gauge wasn't right and the nice boyfriend sweater ends up fitting your 12-year-old nephew just perfectly.
Yes - you may have become a bit bitter, but not broken - then I will simply make the gauge swatch next time. Here a new problem presents itself - you can't hit the gauge! When you hit the number of stitches, you have a completely wrong number of rows, and vice versa. You simply knit in a different way than the designer requires.
We often run into a knitting gauge of 21 sts x 28 rows on 4 mm needles. This corresponds to a ratio of 0.75 - i.e. 21/28 = 0.75. But it is quite possible that your ratio is either above or below this. To find your ratio, divide the number of stitches by the number of rows in 10cm. If your ratio is below 0.75, you knit so-called wide stitches and if it is over, you knit tall stitches. If you knit with a different ratio than the designer, then there is no way around it - you simply have to recalculate the pattern. You can perhaps handle that by focusing on hitting the stitch count, and then keep an eye on knitting a little more or fewer rows. But it can really be almost impossible to hit the fit in both directions.
The end of hopeless projects ending up as UFOs in the back of your wardrobe. Your knits should fit you!