While not a programming item, I had to specially memorize "8 times 7" while in college for a Maths degree. "7 times 8" is 56, obviously, but I could never get it out of my head that "8 times 7" wasn't 48. So, it was like I had a hole in my times tables.
My criteria for good software:
Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?
Actually, 9 is easy. 9×d (where d is a single digit number) is always two digits, the first being d-1 and the second 10-d (or as I was taught, the second being enough so that the two add up to 9). So 9×3 is 27 (2 = 3-1, 2+7=9).