Re: a teaching mistake
You learn more from "Oops" than you do from "Wow". It's a bit rough trying to learn something on the fly when you're trying to teach it, but I run into that all the time.
I coach rowing (and train other rowing coaches) - ok, it's not Aikido, but the thing is, they're in a boat, I'm in another boat, and I can't touch them and guide them through the movement - I have to do it by suggesting exercises or changes in their movements, or occasionally through demonstration/simulation (I'm in a motor boat, they're in racing shells, a minimum of 4 meters away from me). There are a couple of reasons I can't touch them - first - in the boat, they're remote. Second, even when they're on the land on rowing machines, I still can't touch them (without permission, whether they're male or female) because I'm a single male in my 50s and they're usually teenagers or in their 20s. I have to have all kinds of different ways to verbally guide them through learning how to row effectively and efficiently without more contact than a handshake or a pat on the shoulder. (Parenthetically, try running an aikido class without touching any uke, by talking both uke and nage through their movements, and by demonstrating the movement nage should be trying to do, but without an uke.)
Coaching/teaching/training/guiding Aikido is different in that it's done with personal contact and feel, where you attempt to control uke's core/centre through positioning yourself and manipulation of their peripheral bits. I'm just now starting to recognise changes in uke/nage when in contact, and am starting to feel ways that my partners are messing up their techniques in a way that allows me to help them not mess up. When they start getting it, I usually end up on the floor more quickly, more cleanly, and asking "wasn't that easier?" or "That felt a lot more like when (sensei) threw me." oops - the dreaded sempai teaching...
Every person you try to teach/coach/train/guide into the "correct" movement is a different person and you don't share their nervous system, so you don't know at the outset how they'll react or learn. We all learn by trying, doing, repeating, and gradually gaining more sensitivity, smoothness, and ability to recognise the difference between what we're trying to do and what we've done - if you never make mistakes, your improvement is slower.
If you're trying to demonstrate something that you're not that familiar with, and if you've got an odd number of people in the dojo, you could borrow the singleton and pre-practice the movement you're thinking of demonstrating prior to interrupting the class, work out whether or not you understand it well enough to teach it, and then interrupt practice to do the demonstration. If you decide that you don't understand it well enough, wait until sensei returns, or work with some of the others who are at or slightly ahead of your level to see if you can sort out how to do it. If you decide to go ahead and try the technique, let the class know you're still working out how to do it. All that said - if you can't do or adapt the technique "on" someone who doesn't really know how they're supposed to move, you may not know the technique that well. I run into that myself quite frequently...
Better stop before a "quick reply" gets really long