Then there's sempai observing something and wanting to correct it while the student is working on something else.
And students of the same level observing something and wanting to correct it while student in question is working on something else.
And then there's kohai observing something and wanting to correct it while the student is working on something else.
We all agree you should focus on what the teacher wants you to focus on. The rest... depends.
Still, follow the lesson of the teacher...
It takes real effort to get such an environment in your dojo, but when you get it, it works really well. Off course this is a continuous process.
I think it is the responsibility of the teacher to make this very clear to all students. It forces students to think about what the teacher is trying to achieve so they do not distract eachother with other issues. I do correct students that correct others on aspects outside my lesson.
I have encountered people on the tatami that simply did what they wanted to work on regardless of what the teacher is doing. They undermine the entire lesson of the teacher and even worse they often start correcting others on things they work on....very bad.