Introspection and self-improvement can both be powerful tools, but it is best used against much larger problems, many of which will not become apparent by keeping track of writing down every little stupid mistake.
- Procrastinated Task X
- Spent too much time watching CSI marathon instead of working
- Didn't reply to Person Y soon enough.
Those are the kind of mistakes it might be worth keeping track of to see which ones are the kind of "D'oh!" mistakes everyone makes once in a while, and which ones are really bad habits that are holding me back more than I realize.
For example, I should probably be configuring that new web server instead of writing a comment on HN, but I like to take a little while to collect my thoughts and get back in the mood after lunch, which often involves HN. I'm not sure if it's a mistake or not.
But remember that the point is to find the things you do ALL THE TIME. You probably don't dial the phone often in one day, so that wouldn't have been on your list.
About six years ago I was on the phone all day, and yes that was a real problem. In the end it was much more efficient to get a device that dialed a number that was selected on the screen.
So when you consider that the point of the exercise is to identify those things about you and your particular workflow, it makes more sense.
Finally, you can probably appreciate that the "write EVERYthing down" is clearly not something you should do all the time, but rather an interesting mechanism to make you THINK about what you're doing.
That is, get you thinking meta rather than just working heads-down. That is the true lesson.