Here’s another one. It should be quick.
When a student says, “Is it like this?” or the equivalent, I used to err on the side of “yes.” I.e. even if I wasn’t sure exactly what they were saying, but I thought it sounded like it might make sense. I think this was somewhat a function of the fact that I adopted a generally encouraging posture (this is my personality but also a deliberate choice), but it itself was just sort of my reflexive response from within this posture (not a deliberate choice).
It never felt quite right, so over time I trained myself instead to say things like, “I can’t understand what you’re saying but I think you might be onto something, but I’m not sure.” I never had concrete evidence that my original response was doing something unhelpful though.
Now I do. In a recent conversation with one of my teachers, several times I said, “Let me explain back to you what I think you’re saying, and you tell me if it’s right…” And he said, “yes yes yes it’s like…” But I didn’t recognize my attempted explanation in what he seemed to be saying yes to. So, it’s official: this is TOTALLY UNHELPFUL. I’m disoriented; that’s why I asked the question. Unless I come away from your answer feeling sure that you understood me, your “yes” only serves to make me more disoriented.
Take-home lesson. Never say “yes” unless you are sure you have understood fully what the student is saying, and agree with it. As I’ve often discussed before, sometimes a “yes” is inappropriate even then; for example if there’s a danger that the student is trying to foist onto you the work of judging for her or himself. But if you have any doubt, then the “yes” is definitely inappropriate: the encouragement is fake, and the student is left being equally unsure as before, and now also having exhausted the resource of checking with you. Retrospectively the only student who even feels good hearing the “yes” in this situation is the one who is playing a Clever Hans game, and in this case it does him or her the disservice of encouraging the game.
Research in Practice 
I’ve actually never noticed myself having this problem from the student side of things, but I have realized the bad effects of it when teaching. Sometimes I say “yes” too soon, then realize I don’t actually know what I was agreeing to. When I ask the student to clarify further, I often figure out that I was accidentally reinforcing a misconception (usually one which results in getting to the right place but only coincidentally and through faulty logic). I think in general as teachers, we want to believe that our students understand what we’re teaching them, so we have a tendency to say yes too quickly to those types of questions, but it’s important to remind ourselves to slow down and make sure we really understand the students’ thinking.
Somebody once told me that your “take home” lesson is the defining quality of mathematicians outside of academia. But it’s not often understood as such.
Wait I don’t get it, clarify?
Sorry, I was trying to be too short. As I understand it, it’s as follows:
Mathematicians are trained to know when they don’t understand something. And they will often say so and will not pretend to understand something until they do. It’s critical for our work.
In contrast, as I understand it, outside of academia you’ll find a lot of people under pressure to understand complicated issues. They will often react by quickly saying “ok, I understand” even when they really don’t.
At the same time, somebody constantly saying “I don’t understand yet” will, in such an environment, have to justify this. Even if nobody else actually understands either.
Does that make a little more sense?