Someone, somewhere, at some time, worked out that before a knitter starts a project, he/she would be advised to knit a "tension square", the name Australians use for a "gauge swatch".
Most commercial patterns, especially those for articles where size matters, strongly recommend taking the time to knit a tension square.
As a knitting tutor, I wholeheartedly agree. Why spend hours knitting something only to find that after all that "work", the article is the wrong size!
Why then, did I spend five days knitting exclusively on my stealth project only to decide on Day 6 that it was too big? Because, as usual, I was in too much of a hurry to get started on my latest Great Idea. I had all the confidence of a New Project and the fumes of Pure Wool Yarn urging me to just get knitting! I did take the time to prepare a chart for my Great Idea. I knitted and ripped and knitted and ripped; sometimes it felt like I was taking two steps forward and one step back.
But, folks, it wasn't worth it. Late on Day 5 I weighed my yarn. I had 98g left of a 200g ball. What I had knitted was close to the right length I had hoped it to be at this point. At bed time I lay my knitting on the footstool. It was one of the first things I saw on Day 6. And I had to admit, it was just too wide! I wanted something narrower and much longer than I was going to be able to knit with the number of stitches I had!
And so I frogged. This is not the first time I have said it - a gauge swatch is Really. Important. (especially when designing your own knitting) *sigh*