Although talking on the mats during class isn't prohibited in our dojo, it is generally considered impolite as it can interfere with the concentration of other students. When it is necessary - for clarification of a technique, for example - it's kept brief.
The only excpetion would be when a student decides to end his/her life while in the midst of a rather vigorous class. And, of course, if a techinique fails, the ocassional "Down! I said do it!! Now!!!" could be helpful.
Don't you like when someone gives you incorrect instruction?