Six hours on a typo is rough but the lesson you got out of it is worth more than the time you lost, so don't be too hard on yourself. I'd argue the opposite of your last point though. Typos are not some separate category from the "hard" stuff, they're the hard stuff wearing a disguise. A capital I and a lowercase l look the same in half the fonts we use, and that is a design problem, not a you problem. Loops and logic you can learn from a book, but building the habit of slowing down and trusting nothing is the real skill, and it only comes from getting burned. Your 8th graders spotting it in 4 minutes proves the point, fresh eyes have no assumptions. Keep that note on your monitor and next time make the kids debug your code on purpose, you already found the best lesson plan by accident.

Kept using one towel for years and my paint looks fine.