Re: a teaching mistake
The sensei in our dojo do the same thing. It's always nice to be chosen as uke for the private practice and then public demonstration.
With some instructors, I've noticed that when they 'mess up' a demonstration, they'll tend to turn it into another technique, blame the uke but joke about how it was their [the instructor's] fault. It's a good approach, because it not only shows that things (for whatever reason) can go wrong, but also shows how those failures can be salvaged.