I was confused too as to how the origional thread was technical grappling, but maybe big O was just feeling a little more generous than putting it in the mythical BB club, but didn't want us mere mortals clogging it up with attempted comedy.

Anyway, now we're in YMAS here's my 2 cent in giant wall of text form.
(Ive been in all three roles: leader of training when it happened, the know-it-all, and the confused student.)

Me as a Mr. Know It All:
Example 1:
A few months ago I was doing some work in a hall where there was a karate club doing some wrist locks off a cross wrist grab (what aikido would call kosadori nikyo). They were all lined up and I could see the instructor was at the far end of the line giving some help. The pair at my end were a smallish, mid-ranked woman and a large male beginner. They were both having some trouble. She was perplexed by the mans straight arm in the grab (making kote mawashi / nikyo too ineffective to be felt when applied by a beginner, and inviting wakai gatamae / rokyo). He didnt know the motion. I stepped over and asked them if theyd like to do it to me, and let me show them what it felt like. I told the guy to let his elbow bend.
The instructor had a bit of a puzzled look when he say a guy in street clothes giving his students a hand. I felt embaressed about it almost immediately. Later I talked to him saying that those wrist locks are what we train in all the time, and if theyd us to share any information about them Id be happy to help. I obviously heard nothing back from them.

Example 2:
Background: The JJJ guys I play with occasionally are mostly open to change. They have so little newaza that I feel my smattering of experience is the highest in that club. At the instructors request Ive helped him with their version of the bridge and roll more than once. Hes done a very little bjj and is currently training in SL4 kenpo outside of JJJ.
We were practising the bridge and roll when he stopped the class to give a pointer about some SL4 style spookyness in the technique, which he proceeded to do without any bridge. I asked him to do it to me since I see, I am deceived; I feel , I believe. Queue the spookyness doing nothing to me. I give him the same feed and ask him to do it with a bridge and the sweep works. I apologised afterwards, but I still thought it was a dick move on my part.

Being talked to by a Know-it-all:
This has happened to me a few times. Theres one JJJ guy in particular that will start to talk when hes feeling tired. Depending on how much the change is from what were meant to be doing, how much their talking instead of training, and the atmosphere of the dojo Ill use one of a few approaches:
Ask the instructor to come over and clarify what were meant to be doing
Say thats interesting, but I dont think its what were doing now.
Say lets just try it and see what happens.
Say thats interesting, we can talk about that in the pub after training.

Leading training and having people do something other than what Im showing:
Ive had the equivalent of both know-it-all white belts and a know-it-all blue belts on the mat.
Ill do the drill with each of them and say thats what were doing today.
Once I split up a pair and alternated between them, having one air train while I trained with the other, just to stop them giving each other the wrong information.