Do As I Say, Not As I Do
One of the headaches I face as a coach is that I don't have perfect technique. I can demo pretty good technique for most shots, but in live play it's not always perfect - and if I'm not careful, that's what beginners begin to copy.
For example, I have an effective 2200 level forehand loop. If you analyze it piece by piece, the technique isn't bad; it's a bit short, which isn't necessarily a problem. But it's somewhat jerky. No one would mistake it for the smooth loops of your average world-class player. If I'm not careful, students will subconsciously begin to copy the jerkiness of the stroke when that's the one thing about it they shouldn't copy. So I always make a point of mentioning this to students, and often point out the smoother technique of our top players.
I also have a rather flat backhand, when these days most top players use far more topspin. I can demo this, but not that well. So again I often point out the more topspinny backhands of the top players in our club. But when I practice with students, they see my normal flat backhand, and so I have to keep reminding them to go for more topspin and not hit it as flat as I do.