The title of this post is an example of one of the problems that annoys me the most. I feel like I've done good hard teaching to convince my students that when they put a comma into an adjective clause (such as "who hesitates" they must put a comma on both sides of the clause, and, they should do this only when the clause is truly unnecessary information (not needed for the listener/reader to know who). In this case it is necessary information - only those who hesitate are lost - but what has happened? Grammar check has underlined He and who, and politely suggested to the writer that when you use who (or where or which) you really need a comma. This is poor advice. When you need a comma, you need to use a more formal who, where or which, but that's not the same. As you can see from the example above, sometimes you use who and you really shouldn't have a comma.
OK, so the student takes some bad advice, and puts a single comma before who as in the case above. What have we lost? Can I claim that the student has been untaught? Mistaught? Temporarily misled? Made to look like someone who doesn't have a clue about the clause in its entirety?
In fact, I think the situation is quite murky. I have no idea what these students know, or believe (I've seen this several times, with American students as well), and I suspect that all of the above are somewhat true. Students act on murky advice but can't be untaught something they learned with true confidence, i.e. if they really knew what they really wanted they wouldn't go following that grammar-check's flawed advice. Here are some other examples:
The city, where you go to school is important.
Pick the answer, which suits you best and put it in the blank.
The person, who you should talk to is over in admissions.
I find myself more irritated by the omission of the second comma, because I find that our expansive eagerness to find that your sentence is acceptable, and draw meaning from it, allows me to accept the first comma. When I get to the first one, I find it's possible, even though from a more global perspective we can see that it was placed there wrongly, based on wrong advice, and probably doesn't belong there given the writer's meaning. But I read right through that first one, and place blame on the writer for omitting the second one, and my blame is misplaced. The grammar teacher in me is circling a spot in the sentence that is not where the incident occurred. And this brings yet another complication.