I actually find myself at the other end of this, I'm the new guy.
In my class I'm in fact the oldest (it's a children's class and I joined just before my 16th birthday so they kept me in this class understandably until some more get to adult class age), but I'm actually the lowest ranked. I find myself training with some of the higher grades, but I do notice slight things that they are doing wrong. I don't ever think that I've been observing their technique and oh this this and this are all wrong, I'm just a good listener and as I'm new I try to listen as intently as possible to sensei and get all the information in.
However, I have more sense and decency than to "correct" my partner, instead try to utilise what I notice they aren't doing when I perform the technique on them. Usually it comes out that next time they do it they've felt the extra twist or grip that I've done as sensei instructed, and then use this. I think any beginner shouldn't lecture a person trying to teach them, but there are times when the beginner has picked up or remembered something that they have not. (I am by no means saying I do them perfectly, 9 times out of 10 it's me doing something wrong)
In a fairly small class these things are usually cleared up when sensei walks past as they are doing a technique and says "That's not how you do it. THIS *yelp of pain* is how you do it. *grins*" (Does go into more detail but not after having a bit of a laugh
) Best way is sensei.