I had such a nice response to my X-ing X's hat that I decided to write up the pattern. I wanted to reknit it to double-check my work. For the first hat I used leftover Wool-Ease from Christina's Katniss Cowl. I didn't have enough for a second hat so I used a darker gray skein of the same yarn from the same project. I cast on the same number of stitches on the same needles and knit the same hat and look at the results.
So...yeah. I checked the gauge and the first hat came out to 5 stitches per inch. Hat #2 is 4.5 stitches per inch! I thought the dark gray felt a bit thicker when I was knitting it but I never thought to stop and check my gauge because I assumed, incorrectly, that the same brand of yarn on the same needle would yield the same results.
I was wrong.