When I first installed linux I went through a phase of starting everything from a terminal ("lookit me, I'm a hacker!"), and every friggin one of them filled the screen with:
Info: Invalid doohickey. Warning: Something bad is happening, just fyi. Attempting to recover... failed. Limping on valiantly. Warning: Doohickey appears to be growing fangs. Warning: Doohickey is attempting to DOOHICKEY: YOAR COMPUTER ARE BELONGE TO ME NOW Info: Emergency shunting from main loop in order to save humanity. Tell my wife I love her. Thanks for using VLC!
Watching those poor mutilated programs flailing helplessly away at each other in my RAM, I wondered how my computer could function at all. Now, of course, I ignore the pathetic cries for help unless something actually goes wrong.
I think this transition explains the saying, "can do, can't teach".
It's a weird idea, if you think about it. A fat wallet does not make you worse at buying your friends a beer. Why should expertise make you worse at teaching? And yet that often seems to be how it works out.
I've experienced it myself:
Me: So now, type in your friend's email address. Student: It's not working. I'm typing but nothing's happening. Me: Oh. Sorry. First you've got to click on the address field. See how there's no cursor there? Student: Ok, I put the cursor there, but it's still not working. Me: You've got the mouse over the field, now you just have to click. Student: Which letter?
Me: ... and none of your pieces are threatened, so you can develop - Student: You could capture this pawn, couldn't you? Me: No. Student: The queen can move on a slant, right? Me: Oh. Yeah. But the pawn's protected. I'd lose my queen.
I am not making fun of the student in either of these stories. They're both very intelligent people who happen to be meeting these ideas for the first time. They're doing exactly what a learner should be doing - looking at the information they have, and using it to form hypotheses.
The failure is with me. I have used a mouse and captured chess pieces so many times, my brain has abstracted out the logic and registered it as a callback on the appropriate events. I don't even remember the details any more, it just happens. And that hinders my ability to teach.
Beginners need to spend time on details - that's the only way they can get their callbacks registered. A skilled teacher can help them go through that process more quickly, but it can't be skipped.