Re: a teaching mistake
I notice my sensei (almost all of them!) taking a student aside to practice the next technique before they do a formal demonstration. They do not do it all the time, but rather often, especially if they are trying to teach a particular movement throughout the class, they might try several techniques on an uke before deciding which to teach the whole class.
Once in front of the class, this ensures uke has some practice and sensei has made a decision.
It is also fun that the rest of us get a quick preview from the corner of the eye on what is coming next!