But then the answer to "What skill am I actually practicing?" is "How to play really slowly." I'd agree that "Playing slowly to learn to play faster" is probably a smoother upgrade than "playing with mistakes to learn to play more accurately", but that's not obviously true.
I've seen it suggested to practice at normal or faster-than-normal speeds, but only very short segments that you can play for sure without mistakes (one or two measures, repeated 10 times). Anecdotally it's worked pretty well for me, but so have other techniques (playing the whole thing slowly, playing at normal speed with mistakes...)
Of course, then the answer to "What skill am I actually practicing" is "how to play this tiny part of the song (and then stop)"...
See also: Boring Advice Repository, Solved Problems Repository, Grad Student Advice Repository, Useful Concepts Repository, Bad Concepts Repository
I just got back from the July CFAR workshop, where I was a guest instructor. One useful piece of rationality I started paying more attention to as a result of the workshop is the idea of useful questions to ask in various situations, particularly because I had been introduced to a new one:
"What skill am I actually training?"
This is a question that can be asked whenever you're practicing something, but more generally it can also be asked whenever you're doing something you do frequently, and it can help you notice when you're practicing a skill you weren't intending to train. Some examples of when to use this question:
Many of the lessons of the sequences can also be packaged as useful questions, like "what do I believe and why do I believe it?" and "what would I expect to see if this were true?"
I'd like to invite people to post other examples of useful questions in the comments, hopefully together with an explanation of why they're useful and some examples of when to use them. As usual, one useful question per comment for voting purposes.